Fix CI: drop pre-existing PG clusters, use port 5432

Ubuntu 24.04 runners have a pre-installed PG cluster on port 5432.
Drop all existing clusters before creating PG 18 on port 5432.

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

Author: jhf

.github/workflows/regression.yml

@@ -18,7 +18,8 @@ jobs:
 
       - name: Start PostgreSQL 18
         run: |
-          sudo pg_createcluster 18 main --start
+          sudo pg_lsclusters -h | awk '{print $1, $2}' | while read ver name; do sudo pg_dropcluster --stop "$ver" "$name" 2>/dev/null || true; done
+          sudo pg_createcluster 18 main --start --port 5432
           sudo -u postgres createuser -s $(whoami)
 
       - name: Add pg_config to PATH