[release-21.0] copy_state: use a mediumblob instead of a smaller varbinary for lastpk (#18852)#18857

nickvanw merged 1 commit intorelease-21.0vitessio/vitess:release-21.0from

backport-18852-to-release-21.0vitessio/vitess:backport-18852-to-release-21.0Copy head branch name to clipboard