[GITEA] Maintain aspect ratio for auth icons

- When specifying an icon URL for a authentication source, it's forced
to be an width and height of 20px. However this didn't take into account
that icons could have an different aspect ratio of 1:1.
- This patch fixes that by instead using `max-width` and `max-height`
which will respect other aspect ratios.
- Resolves https://codeberg.org/forgejo/forgejo/issues/1234

(cherry picked from commit 3482739637)

web_src/css/base.css

@@ -2289,3 +2289,8 @@ table th[data-sortt-desc] .svg {
   flex-wrap: wrap;
   gap: .25rem;
 }
+
+.oauth-provider-icon {
+  max-height: 20px;
+  max-width: 20px;
+}