The timeseries_union_with.js test is intermittently failing in *retryable_writes_downgrade suites on v6.0 (BF-30780, BF-30781).

In master the test is excluded from the suites via does_not_support_stepdowns tag. It's not clear to me why the test should be excluded (filed SERVER-83992 about improving the test) but given we do so in master, adding the same tag(s) in v6.0 seems like a valid mitigation for the BFs.