GH Actions: ensure linting is done against highest/lowest supported PHP version

`latest` in the matrix will always refer to the latest stable PHP release, which would now be PHP 8.0.

.github/workflows/lint.yml

@@ -18,7 +18,7 @@ jobs:
       matrix:
         php-version:
           - "5.3"
-          - "7.4"
+          - "latest"
 
     steps:
       - name: "Checkout"