ci: drop deprecated NC31 from matrix, enable NC33, stage NC34

Nextcloud 31 reached deprecation (02/2026), so remove it from the integration
matrix. Enable NC33 (previously disabled pending upstream app support) and add
NC34 as a commented, ready-to-enable entry.

- test.yml: nextcloud_version is now [32, 33]; 34 commented. Image pins updated
  to match (32.0.11, 33.0.5 active; 34.0.0 commented). The Renovate customManager
  regex already tracks commented entries, so 34 is digest-managed once present.
- renovate.json: drop the nextcloud-31 pin rule, add nextcloud-34 (/^34\./).

docker-compose.yml already defaults to 32.0.11 (Renovate-pinned to 32.x), so no
change there — the NC31 seen in local runs comes from a shell-exported
NEXTCLOUD_IMAGE override, not the compose default.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

renovate.json

@@ -14,10 +14,6 @@
     },
     {
       "description": "Pin each Nextcloud matrix entry to its own major version",
-      "matchDepNames": ["nextcloud-31"],
-      "allowedVersions": "/^31\\./"
-    },
-    {
       "matchDepNames": ["nextcloud-32"],
       "allowedVersions": "/^32\\./"
     },
@@ -25,6 +21,10 @@
       "matchDepNames": ["nextcloud-33"],
       "allowedVersions": "/^33\\./"
     },
+    {
+      "matchDepNames": ["nextcloud-34"],
+      "allowedVersions": "/^34\\./"
+    },
     {
       "description": "Pin docker-compose.yml Nextcloud image to 32.x",
       "matchPackageNames": ["docker.io/library/nextcloud"],