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

## ⚠️ 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`

custom/conf/app.example.ini

@@ -693,17 +693,13 @@ LEVEL = Info
 ;PULL = 300
 ;GC = 60
 
-;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
-;; Git Reflog timeout in days
-;[git.reflog]
-;ENABLED = true
-;EXPIRATION = 90
-
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Git config options
 ;; This section only does "set" config, a removed config key from this section won't be removed from git config automatically. The format is `some.configKey = value`.
 ;[git.config]
 ;diff.algorithm = histogram
+;core.logAllRefUpdates = true
+;gc.reflogExpire = 90
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;