README.md: update stable release links

(cherry picked from commit 366bebd53aaf6d2ce453590be9c700f8c78f872b)

.github/CONTRIBUTING.md

···
 Follow these steps to backport a change into a release branch in compliance with the [commit policy](https://nixos.org/nixpkgs/manual/#submitting-changes-stable-release-branches).
 
 1. Take note of the commits in which the change was introduced into `master` branch.
-2. Check out the target _release branch_, e.g. `release-20.03`. Do not use a _channel branch_ like `nixos-20.03` or `nixpkgs-20.03`.
+2. Check out the target _release branch_, e.g. `release-20.09`. Do not use a _channel branch_ like `nixos-20.09` or `nixpkgs-20.09`.
 3. Create a branch for your change, e.g. `git checkout -b backport`.
 4. When the reason to backport is not obvious from the original commit message, use `git cherry-pick -xe <original commit>` and add a reason. Otherwise use `git cherry-pick -x <original commit>`. That's fine for minor version updates that only include security and bug fixes, commits that fixes an otherwise broken package or similar. Please also ensure the commits exists on the master branch; in the case of squashed or rebased merges, the commit hash will change and the new commits can be found in the merge message at the bottom of the master pull request.
-5. Push to GitHub and open a backport pull request. Make sure to select the release branch (e.g. `release-20.03`) as the target branch of the pull request, and link to the pull request in which the original change was comitted to `master`. The pull request title should be the commit title with the release version as prefix, e.g. `[20.03]`.
+5. Push to GitHub and open a backport pull request. Make sure to select the release branch (e.g. `release-20.09`) as the target branch of the pull request, and link to the pull request in which the original change was comitted to `master`. The pull request title should be the commit title with the release version as prefix, e.g. `[20.09]`.
 
 ## Reviewing contributions
 

README.md

···
 system, [Hydra](https://hydra.nixos.org/).
 
 * [Continuous package builds for unstable/master](https://hydra.nixos.org/jobset/nixos/trunk-combined)
-* [Continuous package builds for the NixOS 20.03 release](https://hydra.nixos.org/jobset/nixos/release-20.03)
+* [Continuous package builds for the NixOS 20.09 release](https://hydra.nixos.org/jobset/nixos/release-20.09)
 * [Tests for unstable/master](https://hydra.nixos.org/job/nixos/trunk-combined/tested#tabs-constituents)
-* [Tests for the NixOS 20.03 release](https://hydra.nixos.org/job/nixos/release-20.03/tested#tabs-constituents)
+* [Tests for the NixOS 20.09 release](https://hydra.nixos.org/job/nixos/release-20.09/tested#tabs-constituents)
 
 Artifacts successfully built with Hydra are published to cache at
 https://cache.nixos.org/. When successful build and test criteria are

nixos/doc/manual/installation/upgrading.xml

···
     <para>
      <emphasis>Stable channels</emphasis>, such as
      <literal
-    xlink:href="https://nixos.org/channels/nixos-20.03">nixos-20.03</literal>.
+    xlink:href="https://nixos.org/channels/nixos-20.09">nixos-20.09</literal>.
      These only get conservative bug fixes and package upgrades. For instance,
      a channel update may cause the Linux kernel on your system to be upgraded
      from 4.19.34 to 4.19.38 (a minor bug fix), but not from
···
     <para>
      <emphasis>Small channels</emphasis>, such as
      <literal
-    xlink:href="https://nixos.org/channels/nixos-20.03-small">nixos-20.03-small</literal>
+    xlink:href="https://nixos.org/channels/nixos-20.09-small">nixos-20.09-small</literal>
      or
      <literal
     xlink:href="https://nixos.org/channels/nixos-unstable-small">nixos-unstable-small</literal>.
···
  <para>
   When you first install NixOS, you’re automatically subscribed to the NixOS
   channel that corresponds to your installation source. For instance, if you
-  installed from a 20.03 ISO, you will be subscribed to the
-  <literal>nixos-20.03</literal> channel. To see which NixOS channel you’re
+  installed from a 20.09 ISO, you will be subscribed to the
+  <literal>nixos-20.09</literal> channel. To see which NixOS channel you’re
   subscribed to, run the following as root:
 <screen>
 <prompt># </prompt>nix-channel --list | grep nixos
···
 <prompt># </prompt>nix-channel --add https://nixos.org/channels/<replaceable>channel-name</replaceable> nixos
 </screen>
   (Be sure to include the <literal>nixos</literal> parameter at the end.) For
-  instance, to use the NixOS 20.03 stable channel:
+  instance, to use the NixOS 20.09 stable channel:
 <screen>
-<prompt># </prompt>nix-channel --add https://nixos.org/channels/nixos-20.03 nixos
+<prompt># </prompt>nix-channel --add https://nixos.org/channels/nixos-20.09 nixos
 </screen>
   If you have a server, you may want to use the “small” channel instead:
 <screen>
-<prompt># </prompt>nix-channel --add https://nixos.org/channels/nixos-20.03-small nixos
+<prompt># </prompt>nix-channel --add https://nixos.org/channels/nixos-20.09-small nixos
 </screen>
   And if you want to live on the bleeding edge:
 <screen>
···
    kernel, initrd or kernel modules.
    You can also specify a channel explicitly, e.g.
 <programlisting>
-<xref linkend="opt-system.autoUpgrade.channel"/> = https://nixos.org/channels/nixos-20.03;
+<xref linkend="opt-system.autoUpgrade.channel"/> = https://nixos.org/channels/nixos-20.09;
 </programlisting>
   </para>
  </section>