Use docker image mysql:8.0 for all tests

... as the use of MySQL < 8 is deprecated in DBAL 4

Author: relthyg

.github/workflows/tests.yml

@@ -45,7 +45,7 @@ jobs:
 
     services:
       mysql:
-        image: mysql:${{ (matrix.php == '7.3' && '5.7') || '8.0' }}
+        image: mysql:8.0
         env:
           MYSQL_USER: test
           MYSQL_PASSWORD: test