This will resolve BF-26764

The problem is that buildscripts/resmokeconfig/matrix_suites/overrides/multiversion.yml makes a ton of references to last_continuous on the 6.0 branch. But since 6.0 is a major release, it does not need to be compatible with last_continuous (5.3), it only needs to be compatible with last_lts (5.0). This is a fix for now so that we can stop running this testing. The long term fix will be handled in SERVER-70937 / PM-3104

An easier fix is to just remove the retryable_writes_downgrade_last_continuous_gen task from definitions.yml for 6.0.