roachtest: rename clusterImpl to roachprodCluster

Remove the type alias (`roachprodCluster = clusterImpl`) introduced in
the previous PR and rename the struct itself. This gives the
roachprod-backed cluster implementation a self-documenting name,
making it clear which backend is in use and leaving room for future
alternative implementations.

Purely mechanical rename; no behavior changes.

Epic: none
Release note: None

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: golgeek