Remove the ReshardingTest fixture's isMixedVersionCluster() method

There is one remaining usage of ReshardingTest#isMixedVersionCluster() in the resharding_metrics.js test. It won't be needed after the changes from SERVER-57697 and SERVER-57700 have been backported to the 5.0 branch.

As part of this ticket we should also replace the usage of the requires_fcv_51 tag in the resharding_histogram_metrics.js test with entries in etc/backports_required_for_multiversion_tests.yml for SERVER-57700. The original changes in SERVER-57697 and SERVER-57700 weren't guarded by FCV 5.1 so the usage of the requires_fcv_51 tag was incorrect.