Use `[git.config]` for reflog cleaning up (#24958)

Use `[git.config]` for reflog cleaning up (#24958)

Follow
https://github.com/go-gitea/gitea/pull/24860#discussion_r1200589651

Use `[git.config]` for reflog cleaning up, the new options are more
flexible.

*
https://git-scm.com/docs/git-config#Documentation/git-config.txt-corelogAllRefUpdates
*
https://git-scm.com/docs/git-config#Documentation/git-config.txt-gcreflogExpire

## :warning: BREAKING

The section `[git.reflog]` is now obsolete and its keys have been moved
to the following replacements:
- `[git.reflog].ENABLED` → `[git.config].core.logAllRefUpdates`
- `[git.reflog].EXPIRATION` → `[git.config].gc.reflogExpire`