nixos/openssh: Drop deprecated locations

The changelog entry should give a good indication why I don't think this
is necessary anymore.

nixos/doc/manual/release-notes/rl-2305.section.md

···
 
 - The `ssh` client tool now disables the `~C` escape sequence by default. This can be re-enabled by setting `EnableEscapeCommandline yes`
 
+- The `ssh` module does not read `/etc/ssh/ssh_known_hosts2` anymore since this location is [deprecated since 2001](https://marc.info/?l=openssh-unix-dev&m=100508718416162&w=2).
+
+- The openssh module does not read `~/.ssh/authorized_keys2` anymore since this location is [deprecated since 2001](https://marc.info/?l=openssh-unix-dev&m=100508718416162&w=2).
+
 - `podman` now uses the `netavark` network stack. Users will need to delete all of their local containers, images, volumes, etc, by running `podman system reset --force` once before upgrading their systems.
 
 - `git-bug` has been updated to at least version 0.8.0, which includes backwards incompatible changes. The `git-bug-migration` package can be used to upgrade existing repositories.

nixos/modules/programs/ssh.nix

···
       + (if h.publicKey != null then h.publicKey else readFile h.publicKeyFile)
     )) + "\n";
 
-  knownHostsFiles = [ "/etc/ssh/ssh_known_hosts" "/etc/ssh/ssh_known_hosts2" ]
+  knownHostsFiles = [ "/etc/ssh/ssh_known_hosts" ]
     ++ map pkgs.copyPathToStore cfg.knownHostsFiles;
 
 in
···
         description = lib.mdDoc ''
           Files containing SSH host keys to set as global known hosts.
           `/etc/ssh/ssh_known_hosts` (which is
-          generated by {option}`programs.ssh.knownHosts`) and
-          `/etc/ssh/ssh_known_hosts2` are always
-          included.
+          generated by {option}`programs.ssh.knownHosts`) is
+          always included.
         '';
         example = literalExpression ''
           [

nixos/modules/services/networking/ssh/sshd.nix

···
     # https://github.com/NixOS/nixpkgs/pull/10155
     # https://github.com/NixOS/nixpkgs/pull/41745
     services.openssh.authorizedKeysFiles =
-      [ "%h/.ssh/authorized_keys" "%h/.ssh/authorized_keys2" "/etc/ssh/authorized_keys.d/%u" ];
+      [ "%h/.ssh/authorized_keys" "/etc/ssh/authorized_keys.d/%u" ];
 
     services.openssh.extraConfig = mkOrder 0
       ''