docs: remove outdated references to CODEOWNERS

As of b01ca00aed65 (CODEOWNERS: Switch to alternate mechanism,
2024-10-10) and #336261, this repository doesn't use the GitHub
CODEOWNERS file, but instead has its own similar mechanism. Remove and
update references to the CODEOWNERS file in various documentation files.

maintainers/README.md

···
 
 For critical packages, this convention needs to be negotiated with the
 maintainer. A critical package is one that causes mass-rebuild, or where an
-author is listed in the [`CODEOWNERS`](../.github/CODEOWNERS) file.
+author is listed in the [`OWNERS`](../ci/OWNERS) file.
 
 In case of critical security updates, the [security team](https://nixos.org/community/teams/security) might override these
 heuristics in order to get the fixes in as fast as possible.

nixos/README.md

···
 Reviewing process:
 
 - Ensure that the module maintainers are notified.
-  - [CODEOWNERS](https://help.github.com/articles/about-codeowners/) will make GitHub notify users based on the submitted changes, but it can happen that it misses some of the package maintainers.
+  - The continuous integration system will make GitHub notify users based on the submitted changes, but it can happen that it misses some of the package maintainers.
 - Ensure that the module tests, if any, are succeeding.
   - You may invoke OfBorg with `@ofborg test <module>` to build `nixosTests.<module>`
 - Ensure that the introduced options are correct.

pkgs/README.md

···
 - Ensure that the package versioning [fits the guidelines](#versioning).
 - Ensure that the commit text [fits the guidelines](../CONTRIBUTING.md#commit-conventions).
 - Ensure that the package maintainers are notified.
-  - [CODEOWNERS](https://help.github.com/articles/about-codeowners) will make GitHub notify users based on the submitted changes, but it can happen that it misses some of the package maintainers.
+  - The continuous integration system will make GitHub notify users based on the submitted changes, but it can happen that it misses some of the package maintainers.
 - Ensure that the meta field information [fits the guidelines](#meta-attributes) and is correct:
   - License can change with version updates, so it should be checked to match the upstream license.
   - If the package has no maintainer, a maintainer must be set. This can be the update submitter or a community member that accepts to take maintainership of the package.

pkgs/development/haskell-modules/HACKING.md

···
         and work the new member into the `haskell-updates` rotation.
 
     1.  Optionally, have the new member add themselves to the Haskell
-        section in [`CODEOWNERS`](../../../.github/CODEOWNERS).  This
+        section in [`OWNERS`](../../../ci/CODEOWNERS).  This
         will cause them to get pinged on most Haskell-related PRs.