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/web/misc/misc.go

@@ -9,7 +9,6 @@ import (
 	"strconv"
 	"strings"
 
-	"gitea.dev/modules/git"
 	"gitea.dev/modules/httpcache"
 	"gitea.dev/modules/httplib"
 	"gitea.dev/modules/json"
@@ -45,10 +44,6 @@ func SiteManifest(w http.ResponseWriter, req *http.Request) {
 }
 
 func SSHInfo(rw http.ResponseWriter, req *http.Request) {
-	if !git.DefaultFeatures().SupportProcReceive {
-		rw.WriteHeader(http.StatusNotFound)
-		return
-	}
 	rw.Header().Set("content-type", "text/json;charset=UTF-8")
 	_, err := rw.Write([]byte(agit.SshInfoJson))
 	if err != nil {