[BUGFIX] Update github actions to work with current php versions

Related #717

Author: mschwemer

.github/workflows/testing.yaml

@@ -20,9 +20,8 @@ jobs:
       fail-fast: false
       matrix:
         php-version:
-          - 7.2
-          - 7.3
           - 7.4
+          - 8.0
   typoscript-lint:
     name: "TypoScript linter"
     runs-on: ubuntu-20.04
@@ -70,6 +69,5 @@ jobs:
       fail-fast: false
       matrix:
         php-version:
-          - 7.2
-          - 7.3
           - 7.4
+          - 8.0