Revert "PHP 5.6 will fail."

This reverts commit 66014cb.

Author: peterwilsoncc

.github/workflows/phpunit.yaml

@@ -23,7 +23,7 @@ jobs:
       fail-fast: false
       matrix:
         wp-versions: [ '6.3', '6.4', '6.5', '6.6', '6.7', '6.8', 'latest', 'trunk' ]
-        php-versions: ['5.6', '7.4', '8.0', '8.1', '8.2', '8.3', '8.4' ]
+        php-versions: ['7.4', '8.0', '8.1', '8.2', '8.3', '8.4' ]
         exclude:
           - wp-versions: '6.3'
             php-versions: '8.3'