Drop from matrix

Author: barryvdh

.github/workflows/phpunit.yml

@@ -18,10 +18,6 @@ jobs:
           - "lowest"
           - "highest"
         php-version:
-          - "5.6"
-          - "7.0"
-          - "7.1"
-          - "7.2"
           - "7.3"
           - "7.4"
           - "8.0"