set column length for mysql 5.6 compatibility

MySQL 5.6 does not support more than 191 characters when an index is used and when using utf8mb4 as charset.
As a workaround, I define the length of the queue_name field.

src/Symfony/Component/Messenger/Bridge/Doctrine/Transport/Connection.php

@@ -390,6 +390,7 @@ private function addTableToSchema(Schema $schema): void
         $table->addColumn('headers', self::$useDeprecatedConstants ? Type::TEXT : Types::TEXT)
             ->setNotnull(true);
         $table->addColumn('queue_name', self::$useDeprecatedConstants ? Type::STRING : Types::STRING)
+            ->setLength(190) // mysql 5.6 only supports 191 characters on an indexed column in utf8mb4 mode
             ->setNotnull(true);
         $table->addColumn('created_at', self::$useDeprecatedConstants ? Type::DATETIME : Types::DATETIME_MUTABLE)
             ->setNotnull(true);