[PRIVACY] Disable `update_checker` cron tasks

- This is being disabled as it will weekly connect to a domain.
- This only affects existing installations, as new installations will
have a explicit value being written into app.ini due to https://github.com/go-gitea/gitea/pull/21655

(cherry picked from commit cd0b8b6852563118ab8530c01a48bc612efcf58a)
(cherry picked from commit 58d43867cab15a4b93fab6f4b467756e8030fb2d)

services/cron/tasks_extended.go

@@ -146,7 +146,7 @@ func registerUpdateGiteaChecker() {
 	}
 	RegisterTaskFatal("update_checker", &UpdateCheckerConfig{
 		BaseConfig: BaseConfig{
-			Enabled:    true,
+			Enabled:    false,
 			RunAtStart: false,
 			Schedule:   "@every 168h",
 		},