Update rustls-platform-verifier requirement from 0.5 to 0.6 #649

Job	Run time
Check for documentation errors	20s
msrv	1m 8s
Check semver compatibility	1m 22s
Build+test (beta, ubuntu-latest)	2m 12s
Build+test (nightly, ubuntu-latest)	2m 56s
Clippy	1m 16s
features	1m 7s
Build+test (windows-latest, stable)	7m 2s
Build+test (stable, ubuntu-latest)	2m 23s
Format	17s
Build+test (macos-latest, stable)	2m 18s
	22m 21s

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