Drop mysql57 in CI (#684)

Author: staabm

.github/workflows/tests.yml

@@ -21,7 +21,7 @@ jobs:
       matrix:
         include:
           - php-version: "7.4"
-            db-image: 'mysql:5.7'
+            db-image: 'mysql:8.0'
             reflector: "pdo-mysql"
             mode: "recording"
             dependencies: "lowest"