[GH-4613] Use utf8mb4 instead of utf8 for testing connection charset

Author: morozov

tests/Functional/PrimaryReadReplicaConnectionTest.php

@@ -71,7 +71,7 @@ private function createPrimaryReadReplicaConnectionParams(bool $keepReplica = fa
     public function testInheritCharsetFromPrimary(): void
     {
         $charsets = [
-            'utf8',
+            'utf8mb4',
             'latin1',
         ];