ci: update minimum WP version to 6.6 in integration tests

The plugin requires WordPress 6.6 or higher, so the integration tests
need to use 6.6 as the minimum test version instead of 6.4.

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

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: GaryJones

.github/workflows/integration.yml

@@ -38,7 +38,7 @@ jobs:
       matrix:
         include:
           # Check lowest supported WP version, with the lowest supported PHP.
-          - wordpress: '6.4'
+          - wordpress: '6.6'
             php: '8.2'
           # Check latest WP with the highest supported PHP.
           - wordpress: 'latest'