WL#12918: Deprecate mysql_upgrade client

Since WL#12413 was pushed, the mysql_upgrade client is obsolete. Add a
deprecation warning in 8.0.

[prompt:] mysql_upgrade
The mysql_upgrade client is now deprecated. The actions executed by the upgrade
client are now done by the server.
To upgrade, please start the new MySQL binary with the older data directory.
Repairing user tables is done automatically. Restart is not required after upgrade.
The upgrade process automatically starts on running a new MySQL binary with an
older data directory. To avoid accidental upgrades, please use the
--upgrade=NONE option with the MySQL binary. The option --upgrade=FORCE is also
provided to run the server upgrade sequence on demand.
It may be possible that the server upgrade fails due to a number of reasons. In
that case, the upgrade sequence will run again during the next MySQL server
start. If the server upgrade fails repeatedly, the server can be started with
the --upgrade=MINIMAL option to start the server without executing the upgrade
sequence, thus allowing users to manually rectify the problem.