MDEV-23610: Slave user can't run "SHOW SLAVE STATUS" anymore after upgrade to 10.5, mysql_upgrade should take of that

Add a new privilege "SLAVE MONITOR" which will grant user the permission
to execute "SHOW SLAVE STATUS" and "SHOW RELAYLOG EVENTS" commands.

SHOW SLAVE STATUS requires either SLAVE MONITOR/SUPER
SHOW RELAYLOG EVENTS requires SLAVE MONITOR privilege.