Update status and code index after changing the default branch (#27018)

Fix #26723 
Add `ChangeDefaultBranch` to the `notifier` interface and implement it
in `indexerNotifier`. So when changing the default branch,
`indexerNotifier` sends a message to the `indexer queue` to update the
index.

---------

Co-authored-by: techknowlogick <matti@mdranta.net>

models/repo/git.go

@@ -3,7 +3,9 @@
 
 package repo
 
-import "code.gitea.io/gitea/models/db"
+import (
+	"code.gitea.io/gitea/models/db"
+)
 
 // MergeStyle represents the approach to merge commits into base branch.
 type MergeStyle string