Refactor update checker to use AppState (#17387)

We have the `AppState` module now, it can store app related data easily. We do not need to create separate tables for each feature.

So the update checker can use `AppState` instead of a one-row dedicate table.

And the code of update checker is moved from `models` to `modules`.

models/migrations/migrations.go

@@ -351,9 +351,11 @@ var migrations = []Migration{
 	// v198 -> v199
 	NewMigration("Add issue content history table", addTableIssueContentHistory),
 	// v199 -> v200
-	NewMigration("Add remote version table", addRemoteVersionTable),
+	NewMigration("No-op (remote version is using AppState now)", addRemoteVersionTableNoop),
 	// v200 -> v201
 	NewMigration("Add table app_state", addTableAppState),
+	// v201 -> v202
+	NewMigration("Drop table remote_version (if exists)", dropTableRemoteVersion),
 }
 
 // GetCurrentDBVersion returns the current db version