[v13.0/forgejo] chore: update go target language version to v1.25.0 (#9827)

**Backport:** https://codeberg.org/forgejo/forgejo/pulls/9822

Requirement for upcoming work to use APIs added to the standard library in Go 1.25.  I've broken this out into a separate PR to ensure there are no related test failures.

## Checklist

The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).

### Tests

- I added test coverage for Go changes...
  - [ ] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Co-authored-by: Mathieu Fenniak <[email protected]>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/9827
Reviewed-by: 0ko <[email protected]>
Co-authored-by: forgejo-backport-action <[email protected]>
Co-committed-by: forgejo-backport-action <[email protected]>

Author: 2 people

go.mod

@@ -1,6 +1,6 @@
 module forgejo.org
 
-go 1.24.0
+go 1.25.0
 
 toolchain go1.25.3