chore(CI): Adjust testing matrix for Nextcloud 26 on main

Signed-off-by: Joas Schilling <coding@schilljs.com>

.github/workflows/phpunit-mysql.yml

@@ -39,7 +39,7 @@ jobs:
     strategy:
       matrix:
         php-versions: ['8.1']
-        server-versions: ['master']
+        server-versions: ['master', 'stable26']
 
     services:
       mysql: