[5.7][Test] Fix protocol_conformance_collision.swift.
Use 2021 OS versions instead of far-future OS versions. We have versions with the fix now, so we don't need the far-future versions. Using them breaks things when building against a newer SDK and running on an older OS, because the system assumes stuff on the newer OS is available when it isn't.
(cherry picked from commit 12203217560bc6f60947b0cd56162442420105f0)
