chore(deps): bump postgres docker tag to v15

Signed-off-by: Renovate Bot <bot@renovateapp.com>

.github/workflows/test.yml

@@ -86,7 +86,7 @@ jobs:
           --health-timeout=5s
           --health-retries=3
       postgres-service:
-        image: postgres:14
+        image: postgres:15
         env:
           POSTGRES_USER: nextcloud
           POSTGRES_DB: nextcloud