Update debian maintainer cassandra public keys with latest changes

Current debian build failing [1].

10:45:28  W: GPG error: https://downloads.apache.org/cassandra/debian 40x InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5E85B9AE0B84C041

This is updated out of the debian repository keys file [2].

Local tests concur it's working [3]

[1] https://jenkins.softwareheritage.org/job/debian/job/packages/job/DSTO/job/gbp-buildpackage/372/console

[2] https://downloads.apache.org/cassandra/KEYS

[3] https://forge.softwareheritage.org/P1103