GH Actions: limit the use of MySQL 8.0 to PHP 7.4 or higher

... as based on the test output, there are issues in WP Core which prevent the tests from being run against MySQL 8.0 using PHP 7.3 or lower.

These issues have been reported upstream. See: https://core.trac.wordpress.org/ticket/52496

Author: jrfnl

.github/workflows/test.yml

@@ -109,7 +109,11 @@ jobs:
         # See: https://core.trac.wordpress.org/ticket/34692
         # WP 5.4 is the first WP version which largely supports MySQL 8.0.
         # See: https://core.trac.wordpress.org/ticket/49344
-        image: mysql:${{ ( matrix.wp < 4.5 && '5.6' ) || ( matrix.wp < 5.4 && '5.7' ) || '8.0' }}
+        # During the setting up of these tests, it became clear that MySQL 8.0
+        # in combination with PHP < 7.4 is not properly/sufficiently supported
+        # within WP Core.
+        # See: https://core.trac.wordpress.org/ticket/52496
+        image: mysql:${{ ( matrix.wp < 4.5 && '5.6' ) || ( (matrix.wp < 5.4 || matrix.php < 7.4) && '5.7' ) || '8.0' }}
         env:
           MYSQL_ALLOW_EMPTY_PASSWORD: false
         ports: