openssh: disable support for DSA keys by default

DSA signature is being deprecated by upstream as noted in the 9.7
release notes: https://www.openssh.com/txt/release-9.7

An attribute is added to allow users to restore support while it is
still possible.

nixos/doc/manual/release-notes/rl-2405.section.md

@@ -201,6 +201,9 @@ The pre-existing [services.ankisyncd](#opt-services.ankisyncd.enable) has been m
   "mysecret"` becomes `services.aria2.rpcSecretFile = "/path/to/secret_file"`
   where the file `secret_file` contains the string `mysecret`.
 
+- `openssh`, `openssh_hpn` and `openssh_gssapi` are now compiled without support for the DSA signature algorithm as it is being deprecated upstream. Users still relying on DSA keys should consider upgrading
+  to another signature algorithm. It is however possible, for the time being, to restore the DSA keys support using `override` to set `dsaKeysSupport = true`.
+
 - `buildGoModule` now throws error when `vendorHash` is not specified. `vendorSha256`, deprecated in Nixpkgs 23.11, is now ignored and is no longer a `vendorHash` alias.
 
 - Invidious has changed its default database username from `kemal` to `invidious`. Setups involving an externally provisioned database (i.e. `services.invidious.database.createLocally == false`) should adjust their configuration accordingly. The old `kemal` user will not be removed automatically even when the database is provisioned automatically.(https://github.com/NixOS/nixpkgs/pull/265857)

pkgs/tools/networking/openssh/common.nix

@@ -30,6 +30,7 @@
 , nixosTests
 , withFIDO ? stdenv.hostPlatform.isUnix && !stdenv.hostPlatform.isMusl
 , withPAM ? stdenv.hostPlatform.isLinux
+, dsaKeysSupport ? false
 , linkOpenssl ? true
 }:
 
@@ -84,6 +85,7 @@ stdenv.mkDerivation {
     "--with-libedit=yes"
     "--disable-strip"
     (lib.withFeature withPAM "pam")
+    (lib.enableFeature dsaKeysSupport "dsa-keys")
   ] ++ lib.optional (etcDir != null) "--sysconfdir=${etcDir}"
     ++ lib.optional withFIDO "--with-security-key-builtin=yes"
     ++ lib.optional withKerberos (assert libkrb5 != null; "--with-kerberos5=${libkrb5}")