downloads repos - prep for new signing key

Last release announcing the coming of the new key:

https://mariadb.com/kb/en/mariadb-10-5-18-release-notes/#repositories

The downloads infrastructure is referring to the old key:

With https://mariadb.org/download/?t=repo-config,

debian instructions reference 'https://mariadb.org/mariadb_release_signing_key.asc'
rpm instructions reference $MIRROR/yum/RPM-GPG-KEY-MariaDB rather than the RPM-GPG-KEY-MariaDB_v2