feat(api): add last_commit_when to contents response (#7418)

- Add a new field `last_commit_when` to the `ContentResponse` type, which is populated with the last commit's commiter date. This can be used to determine when the last edit of the content was.
- This field is compatible with what Gitea will likely add, https://github.com/go-gitea/gitea/pull/32921. There's no field for this information in the Github API, so no way to be compatible with that (this API endpoint is otherwise fully compatible with Github's API).
- Ref: gitnex/GitNex#1225
- Integration test adjusted. The API tests cannot test the actual output, as `testify` tries to 'deep equal' the `time.Time` structs which will differ due how the `time.Time` struct is created. Unit tests still verify the output.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7418
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

templates/swagger/v1_json.tmpl

@@ -22022,6 +22022,11 @@
           "type": "string",
           "x-go-name": "LastCommitSHA"
         },
+        "last_commit_when": {
+          "type": "string",
+          "format": "date-time",
+          "x-go-name": "LastCommitWhen"
+        },
         "name": {
           "type": "string",
           "x-go-name": "Name"