Add PHP 8.6 to the CI matrix (#7)

Author: Copilot

.github/workflows/coding-standard-test.yml

@@ -17,6 +17,7 @@ jobs:
           - "8.3"
           - "8.4"
           - "8.5"
+          - "8.6"
     uses: ./.github/workflows/coding-standard.yml
     with:
       php-version: ${{ matrix.php-version }}