MDEV-26230: mysql_upgrade fails to load type_mysql_json due to insufficient

maturity level

Fix: Bumped maturity of the mysql_json plugin to gamma.

plugin/type_mysql_json/type.cc

@@ -211,6 +211,6 @@ maria_declare_plugin(type_mysql_json)
   NULL,
   NULL,
   "0.1",
-  MariaDB_PLUGIN_MATURITY_BETA
+  MariaDB_PLUGIN_MATURITY_GAMMA
 }
 maria_declare_plugin_end;