ScyllaDB release: version 1.6.6

ScyllaDB 1.6.6 is a bugfix release for the 1.6 stable branch, addressing several issues such as a minor memory leak caused by using User Defined Types (UDT) and unexpected exits following alterations to table replication factors. This release remains backward compatible and supports rolling upgrades, though users are encouraged to update to the more recent stable release, ScyllaDB 1.7.2. Notably, the update process from version 1.5.x to 1.6.6 requires attention to changes in metric names. The release also highlights performance impacts related to nodetool operations, like major compaction and cleanup, which could affect disk space and client request performance. Users are advised to report any issues encountered during the upgrade process.