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`

tests/integration/git_general_test.go

@@ -814,11 +814,6 @@ func doCreateAgitFlowPull(dstPath string, ctx *APITestContext, headBranch string
 	return func(t *testing.T) {
 		defer tests.PrintCurrentTest(t)()
 
-		// skip this test if git version is low
-		if !git.DefaultFeatures().SupportProcReceive {
-			return
-		}
-
 		gitRepo, err := git.OpenRepositoryLocal(t.Context(), dstPath)
 		require.NoError(t, err)