fix(git): leave reflog expiry to git's defaults (#39703) (#39711)

Backport #39703 by @silverwind

Gitea's default `gc.reflogExpire = 90` is parsed by git as
`1990-<month>-<day>`, currently `1990-10-09`, so reachable reflog
entries never expired.

Since git 2.54, auto maintenance runs its reflog-expire task in the
foreground of every push, and its trigger ignores reachability, so busy
repos rerun `git reflog expire --all` on every push without pruning
anything, stalling large repos for over a minute.

- Stop setting `gc.reflogExpire` so git's own defaults apply, and remove
the `90` written by earlier versions
- Treat the legacy `[git.reflog] EXPIRATION` as days, as documented

The next push to each repo prunes the accumulated entries once.

Fixes: https://github.com/go-gitea/gitea/issues/39693

Co-authored-by: silverwind <me@silverwind.io>

custom/conf/app.example.ini

@@ -810,7 +810,6 @@ LEVEL = Info
 ;[git.config]
 ;diff.algorithm = histogram
 ;core.logAllRefUpdates = true
-;gc.reflogExpire = 90
 
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