Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/299529

**Problem**

TagsFinder interface is not compatible with other Finders (like
BranchesFinder, Repositories::TreeFinder). It prevents us from using a
keyset pagination classes.

**Solution**

* Restore TagsFinder interface to return a list of tags.
* Raise an exception when Gitaly is not available
* Handle TagsFinder exceptions
* Return 503 error for tags API endpoint when Gitaly is unavailable

Changelog: changed