Migrations are blocked during 8.x ↔ 9.0 FCV transitions (upgrade and downgrade), unconditionally and without a feature flag.

migrations_with_mixed_fcv.js asserts that migrations succeed while shards are in mixed FCV states during upgrade/downgrade. That no longer matches server behavior, so we are temporarily disabling the test. We will re-enable it once 9.0 becomes last-LTS.