A runner can either belong to an owner (user, organization) or a repository, not both. While `CreateRunner()` enforces that, `UpdateRunner()` does not, which leads to bugs. With this change, `UpdateRunner()` rejects runners that have _both_ fields set to prevent unexpected ownership changes.
Resolves https://codeberg.org/forgejo/forgejo/issues/12106 and makes https://codeberg.org/forgejo/forgejo/pulls/12117 obsolete.
### Tests for Go changes
- I added test coverage for Go changes...
- [x] in their respective `*_test.go` for unit tests.
- [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I ran...
- [x] `make pr-go` before pushing
### 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] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [ ] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/13262
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>
