[5.9] Downgrade priors version mismatch warning

[keith]

As a user when you're changing between versions of these files there's
nothing you could do to avoid invalidation so this warning feel
superfluous

(cherry picked from commit e98ddbd / #1396)

[keith]

@swift-ci please test

[nkcsgexi]

Probably it's too late to merge 5.9 PRs.