kvserver, roachtest: run follower_reads roachtest in mixed-version cluster

[aayushshah15]

In 21.1 we're re-working how closed timestamps are propagated throughout the system. A critical challenge to this is ensuring that follower reads continue to work in a mixed-version cluster, as some nodes will still be using the existing side-transport mechanism.

We should add a variant of the existing follower reads roachtest that exercises its workload on a mixed-version cluster.