fix(nextcloud): enable mysql.utf8mb4 to fix collation errors with MySQL 8.4

MySQL 8.4 remapped `utf8` to mean `utf8mb4`, but Nextcloud without this
config sends `COLLATE UTF8_general_ci` (a utf8mb3-only collation) in
queries, causing SQLSTATE[42000] errors that broke occ commands and sync.

Also removed stale `Work 🎯.csv` whose emoji filename was stripped in the
DB filecache (stored as `Work .csv`), causing permanent sync errors.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

stacks/nextcloud/chart_values.yaml

@@ -67,6 +67,11 @@ nextcloud:
         'loglevel' => 2,
         'mail_smtpdebug' => false,
       );
+    zzz-mysql.config.php: |
+      <?php
+      $CONFIG = array(
+        'mysql.utf8mb4' => true,
+      );
   phpConfigs:
     zzz-custom.ini: |
       max_execution_time = 300