[FEATURE] Add PHP 7.2 to the CI test matrix

After the downgrade to PHPUnit 8.5, we now can run the tests on
PHP 7.2 as well, ensuring that everything works fine.

Author: oliverklee

.github/workflows/tests.yml

@@ -12,6 +12,12 @@ jobs:
       fail-fast: false
       matrix:
         include:
+          - typo3-version: "^9.5"
+            php-version: "7.2"
+            composer-flags: ""
+          - typo3-version: "^9.5"
+            php-version: "7.2"
+            composer-flags: " --prefer-lowest"
           - typo3-version: "^9.5"
             php-version: "7.3"
             composer-flags: ""