gogs: mark as insecure

gogs is affected by the recent vulnerabilities reported by the forgejo
team. There is little/no real development activities in the gogs
repository and the upstream maintainers do not seem to have
acknowledged the security issues.

pkgs/applications/version-management/gogs/default.nix

···
     license = licenses.mit;
     maintainers = [ maintainers.schneefux ];
     mainProgram = "gogs";
+    knownVulnerabilities = [ ''
+      Gogs has known unpatched vulnerabilities and upstream maintainers appears to be unresponsive.
+
+      More information can be found in forgejo's blogpost: https://forgejo.org/2023-11-release-v1-20-5-1/
+
+      You might want to consider migrating to Gitea or forgejo.
+    '' ];
   };
 }