enhance!: raise minimum git version to 2.34 (#39565)

Raise the minimum git version to 2.34, the version in Ubuntu 22.04,
Debian 12 and RHEL 8 ship newer, and remove the fallbacks it makes
obsolete.

- Always enable AGit
- Use `diff --skip-to` and `apply -3` unconditionally
- Set the default branch of new repos and wikis via `git init
--initial-branch`
- Detect rebase conflicts via `REBASE_HEAD`

routers/private/serv.go

@@ -247,7 +247,7 @@ func ServCommand(ctx *context.PrivateContext) {
 		// AGit flow needs to write its own ref when the doer has "reader" permission (allowing to create PR).
 		// The real permission check is done in HookPreReceive (routers/private/hook_pre_receive.go).
 		// Here it should relax the permission check for "git push (git-receive-pack)", but not for others like LFS operations.
-		if git.DefaultFeatures().SupportProcReceive && unitType == unit.TypeCode && verb == git.CmdVerbReceivePack {
+		if unitType == unit.TypeCode && verb == git.CmdVerbReceivePack {
 			mode = perm.AccessModeRead
 		}