Fix CI by relaxing our MSRV constraint #2122

Job	Run time
macos-aarch64	48s
x86_64_linux_native_builds (x86_64-unknown-linux-musl)	1m 46s
x86_64_linux_native_builds (x86_64-unknown-linux-gnu)	1m 32s
macos	3m 0s
solaris (x86_64-pc-solaris)	6m 29s
rustfmt	8s
minver	17s
aarch64_linux_native_builds (aarch64-unknown-linux-musl)	6h 0m 16s
aarch64_linux_native_builds (aarch64-unknown-linux-gnu)	1m 22s
tier3 (x86_64-unknown-dragonfly)	1m 5s
tier3 (armv7-unknown-linux-uclibceabihf)	1m 12s
tier3 (x86_64-unknown-haiku)	1m 2s
tier3 (x86_64-unknown-openbsd)	1m 1s
cross_compiling	0s
redox	0s
msrv	0s
cross	0s
	6h 19m 58s

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