feat: add automated PostgreSQL version check workflow

[jor2]

Summary

Adds a GitHub Actions workflow that automatically checks for PostgreSQL version updates from IBM Cloud Databases and creates PRs when changes are detected.

How It Works

• Runs daily at 6 AM UTC (configurable)
• Fetches available PostgreSQL versions from IBM Cloud API
• Extracts current versions from variables.tf (no manual maintenance needed)
• Creates a PR if new versions are available or old versions are deprecated
• Updates:
  • variables.tf - validation block
  • ibm_catalog.json - version options
  • tests/pr_test.go - latestVersion constant

Dependencies

This workflow uses a reusable workflow from common-pipeline-assets:

• PR: feat: add ICD version update reusable workflow common-pipeline-assets#830

Testing

1. Merge the common-pipeline-assets PR first (or test with the feature branch reference)
2. Run the workflow manually with "dry run" enabled to verify detection works
3. Once validated, update the workflow to reference @main

Required Secrets

• IBMCLOUD_API_KEY - IBM Cloud API key with access to Cloud Databases service

[jor2]

#767
terraform-ibm-modules/common-pipeline-assets#830