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/hook_pre_receive.go

@@ -108,7 +108,7 @@ func HookPreReceive(ctx *gitea_context.PrivateContext) {
 			preReceiveBranch(ourCtx, oldCommitID, newCommitID, refFullName)
 		case refFullName.IsTag():
 			preReceiveTag(ourCtx, refFullName)
-		case git.DefaultFeatures().SupportProcReceive && refFullName.IsFor():
+		case refFullName.IsFor():
 			preReceiveFor(ourCtx, refFullName)
 		default:
 			ourCtx.assertCanWriteRef(refFullName)

routers/private/hook_proc_receive.go

@@ -9,7 +9,6 @@ import (
 
 	issues_model "gitea.dev/models/issues"
 	user_model "gitea.dev/models/user"
-	"gitea.dev/modules/git"
 	"gitea.dev/modules/private"
 	"gitea.dev/modules/web"
 	"gitea.dev/services/agit"
@@ -19,10 +18,6 @@ import (
 // HookProcReceive proc-receive hook - only handles agit Proc-Receive requests at present
 func HookProcReceive(ctx *gitea_context.PrivateContext) {
 	opts := web.GetForm[*private.HookOptions](ctx)
-	if !git.DefaultFeatures().SupportProcReceive {
-		ctx.Status(http.StatusNotFound)
-		return
-	}
 	if !loadContextDoerPermission(ctx, opts.UserID, opts.UserExtDoerData) {
 		return
 	}

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
 		}