Bump minimum WordPress version to 6.4

Update minimum supported WordPress version to 6.4.

Changes:
- Plugin header: Update/add 'Requires at least: 6.4'
- README: Update minimum WP to 6.4, tested up to 6.8
- PHPCS config: Update minimum_supported_wp_version to 6.4
- Workflow: Update test matrix minimum to 6.4

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

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

Author: GaryJones

.github/workflows/php-tests.yml

@@ -19,7 +19,7 @@ jobs:
         include:
           # Check lowest supported WP version, with the lowest supported PHP.
           - php: '8.0'
-            wp: '6.0'
+            wp: '6.4'
             allowed_failure: false
           # Check latest WP with the highest supported PHP.
           - php: 'latest'