use mysql 8 syntax for ON DUPLICATE KEY UPDATE

use mysql 8 syntax for ON DUPLICATE KEY UPDATE

Added support for MySQL 8's new ``AS <name> ON DUPLICATE KEY`` syntax when
using :meth:`_mysql.Insert.on_duplicate_key_update`, which is required for
newer versions of MySQL 8 as the previous syntax using ``VALUES()`` now
emits a deprecation warning with those versions. Server version detection
is employed to determine if traditional MariaDB / MySQL < 8 ``VALUES()``
syntax should be used, vs. the newer MySQL 8 required syntax. Pull request
courtesy Caspar Wylie.

Fixes: #8626
Closes: #9210
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/9210
Pull-request-sha: 1c8dfbf0b4c439d9ca2c194524c47eb7239ee3c5

Change-Id: I42c463837af06bc15b60c534159804193df07f02