Update test matrix for PHP and WordPress versions

Author: josephfusco

.github/workflows/test-plugin.yml

@@ -11,8 +11,8 @@ jobs:
     runs-on: ubuntu-22.04
     strategy:
       matrix:
-        php: [ '8.3', '8.2', '8.1' ]
-        wordpress: [ '6.9', '6.8' ]
+        php: [ '8.3', '8.1' ]
+        wordpress: [ '6.9', '6.8', '6.7', '6.6' ]
       fail-fast: false
     name: WordPress ${{ matrix.wordpress }}, PHP ${{ matrix.php }}
     steps: