FindBoost: Do not warn about now-supported version 1.78

In commit d176ff71 (FindBoost: Add support for Boost 1.78, 2021-12-15)
we forgot to update the future-version check.

Issue: #23016
36 jobs for !6819 with FindBoost-1.78 in 11 minutes and 48 seconds (queued for 2 seconds)
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