Support disabling database auto migration (#22053)

Gitea will migrate the database model version automatically, but it
should be able to be disabled and keep Gitea shutdown if the version is
not matched.

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -444,6 +444,7 @@ The following configuration set `Content-Type: application/vnd.android.package-a
 - `MAX_OPEN_CONNS` **0**: Database maximum open connections - default is 0, meaning there is no limit.
 - `MAX_IDLE_CONNS` **2**: Max idle database connections on connection pool, default is 2 - this will be capped to `MAX_OPEN_CONNS`.
 - `CONN_MAX_LIFETIME` **0 or 3s**: Sets the maximum amount of time a DB connection may be reused - default is 0, meaning there is no limit (except on MySQL where it is 3s - see #6804 & #7071).
+- `AUTO_MIGRATION` **true**: Whether execute database models migrations automatically.
 
 Please see #8540 & #8273 for further discussion of the appropriate values for `MAX_OPEN_CONNS`, `MAX_IDLE_CONNS` & `CONN_MAX_LIFETIME` and their
 relation to port exhaustion.