chore: remove `mbstring.func_overload` from `.user.ini`

Deprecated in PHP 7.2
Removed in PHP 8.0
https://www.php.net/manual/en/mbstring.configuration.php#ini.mbstring.func-overload

Signed-off-by: Josh <josh.t.richards@gmail.com>

.user.ini

@@ -1,7 +1,9 @@
 ; SPDX-FileCopyrightText: 2016-2024 Nextcloud GmbH and Nextcloud contributors
 ; SPDX-FileCopyrightText: 2014-2016 ownCloud, Inc.
 ; SPDX-License-Identifier: AGPL-3.0-only
-mbstring.func_overload=0
+;
+; NOTE: PHP caches this file for 300 seconds by default
+;
 always_populate_raw_post_data=-1
 default_charset='UTF-8'
 output_buffering=0