gitea: 1.10.3 -> 1.11.3

Updates `gitea` to the latest version available[1]. Also ensured that
upgrading from `gitea-1.9` (used on NixOS 19.09) to `1.11.3` works
seamlessly.

The derivation required a few more changes this time since `gitea` uses
`npm` now to build the frontend[2]. When using the default tarball from
GitHub, we'd have to build the frontend manually. By fetching a custom
tarball published on every release, we get a prebuilt frontend
(as it was the case on previous versions) and build the backend only from
source.

Co-authored-by: kolaente <k@knt.li>
Closes #80175

[1] https://github.com/go-gitea/gitea/releases/tag/v1.11.3
[2] https://github.com/go-gitea/gitea/issues/10253