Sign key backup with cross-signing key on upgrade
Sign key backup with cross-signing key on upgrade

Add a signature from the cross-signing master key to the key
backup when upgrading the key backup into cross-signing.

For https://github.com/vector-im/riot-web/issues/11747

matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk