Use redpanda-ansible-collection git branch for GCP AR URL testing

Point redpanda.cluster collection at the devprod-4109-gcp-ar-urls branch
instead of Galaxy, so the Buildkite pipeline validates GCP AR URLs before
the collection is published. Also removes ansible/group_vars/all.yml which
was a temporary URL override now superseded by the collection change.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: PrzemekZglinicki

requirements.yml

@@ -1,7 +1,9 @@
 collections:
   - name: community.general
   - name: redpanda.cluster
-    type: galaxy
+    type: git
+    source: https://github.com/redpanda-data/redpanda-ansible-collection.git
+    version: devprod-4109-gcp-ar-urls
   - name: ansible.posix
   - name: grafana.grafana
     version: 5.6.0