Rename history store statistic to better describe its use case.

As part of WT-6811 we've modified mixed mode timestamp operations, particularity with older readers in mind. However the SERVER tickets that are the drivers for that work also suggest that a similar situation is occurring for standalone (non-timestamped) mongodb instances.

The intention of this work is to ensure that the scenario seen in WT-6811 isn't, or is, coming into play for standalone instances.