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.

custom/conf/app.example.ini

@@ -403,6 +403,9 @@ LOG_SQL = false ; if unset defaults to true
 ;;
 ;; Database maximum number of open connections, default is 0 meaning no maximum
 ;MAX_OPEN_CONNS = 0
+;;
+;; Whether execute database models migrations automatically
+;AUTO_MIGRATION = true
 
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