Add PostgreSQL 18 stable to test matrix

- Update from 18beta1 to stable PostgreSQL 18
- Test complete PostgreSQL ecosystem: 13-18
- PostgreSQL 18 uses same pg_stat_checkpointer as 17

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: NikolayS

.github/workflows/test.yml

@@ -12,7 +12,7 @@ jobs:
 
     strategy:
       matrix:
-        postgres-version: ['13', '14', '15', '16', '17', '18beta1']
+        postgres-version: ['13', '14', '15', '16', '17', '18']
       fail-fast: false
 
     services: