bump MSRV to 1.70.0 #256

Job	Run time
e2e (s3s-proxy, mint)	7m 16s
e2e (s3s-proxy, mint)	7m 16s
develop (1.70.0)	4m 50s
develop (1.70.0)	4m 50s
develop (stable)	3m 58s
develop (stable)	3m 58s
develop (nightly)	5m 9s
develop (nightly)	5m 9s
cross-test (stable, ubuntu-latest)	2m 40s
cross-test (stable, ubuntu-latest)	2m 40s
cross-test (stable, windows-latest)	5m 33s
cross-test (stable, windows-latest)	5m 33s
cross-test (stable, macos-latest)	4m 28s
cross-test (stable, macos-latest)	4m 28s
	1h 7m 48s

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