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`

modules/git/remote.go

@@ -66,7 +66,7 @@ func (err *ErrInvalidCloneAddr) Unwrap() error {
 
 // IsRemoteNotExistError checks the prefix of the error message to see whether a remote does not exist.
 func IsRemoteNotExistError(err error) bool {
-	return gitcmd.IsStderr(err, gitcmd.StderrNoSuchRemote1, gitcmd.StderrNoSuchRemote2)
+	return gitcmd.IsStderr(err, gitcmd.StderrNoSuchRemote)
 }
 
 // ParseRemoteAddr checks if given remote address is valid,