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/git.go

@@ -24,12 +24,11 @@ import (
 	"github.com/hashicorp/go-version"
 )
 
-const RequiredVersion = "2.25.0" // the minimum Git version required
+const RequiredVersion = "2.34.0" // the minimum Git version required
 
 type Features struct {
 	gitVersion *version.Version
 
-	SupportProcReceive         bool           // >= 2.29
 	SupportHashSha256          bool           // >= 2.42, SHA-256 repositories no longer an ‘experimental curiosity’
 	SupportedObjectFormats     []ObjectFormat // sha1, sha256
 	SupportCheckAttrOnBare     bool           // >= 2.40
@@ -79,7 +78,6 @@ func loadGitVersionFeatures() (*Features, error) {
 	}
 
 	features := &Features{gitVersion: ver}
-	features.SupportProcReceive = features.CheckVersionAtLeast("2.29")
 	features.SupportHashSha256 = features.CheckVersionAtLeast("2.42")
 	features.SupportedObjectFormats = []ObjectFormat{Sha1ObjectFormat}
 	if features.SupportHashSha256 {