Mark gssapi plugin as stable. No open bug reports, and no further work...

Mark gssapi plugin as stable. No open bug reports, and no further work planned, thus stable is accurate

plugin/auth_gssapi/server_plugin.cc

@@ -177,7 +177,7 @@ maria_declare_plugin(gssapi_server)
   NULL,                                  /* status variables */
   system_variables,                      /* system variables */
   "1.0",
-  MariaDB_PLUGIN_MATURITY_BETA
+  MariaDB_PLUGIN_MATURITY_STABLE
 }
 maria_declare_plugin_end;