roachtest: automatically take app tenant debug zips in mixed version test or all roachprod clusters

Recently, I couldn't get to the bottom of mixed version test failure in https://github.com/cockroachdb/cockroach/issues/150772 because it required an app tenant debug zip to debug possible contention in the tenant key space. While the tenant logs existed the artifacts on TC, the debug zip did not.

A possible hacky solution is to modify the MVT teardown [func](https://github.com/msbutler/cockroach/blob/butler-stats-diff-estimates-2/pkg/cmd/roachtest/roachtestutil/mixedversion/runner.go#L360), or perhaps there's a more general solution to loop over available tenants within `c.FetchDebugZip`[ here](https://github.com/msbutler/cockroach/blob/butler-stats-diff-estimates-2/pkg/cmd/roachtest/cluster.go#L1395).

This feels like a nice starter project. With some direction, i can also do it.

Jira issue: CRDB-53538

Start with the mixed-version teardown function in pkg/cmd/roachtest/roachtestutil/mixedversion/runner.go and the FetchDebugZip implementation in pkg/cmd/roachtest/cluster.go. Trace how available tenants and roachprod clusters are handled, then verify that app tenant debug zips are collected and appear with the other test artifacts for mixed-version tests or all applicable clusters.

Written by the indexing model from the issue text.