Add rust-version, so that our MSRV is enforced by cargo: #538

Job	Run time
lint	4m 52s
check-minimal-versions	4m 1s
test (7.5.1, 3.5)	6h 0m 23s
check (macos-11.0)	8m 27s
test (5.3.1, 2.3)	4h 29m 45s
check (windows-2019, cmake-build,libz-static,curl-static, cmake-build,libz-static,curl-static)	8m 19s
test (5.0.3, 2)	6h 0m 27s
check (ubuntu-20.04, tracing)	5m 5s
test (4.1.3, 1.1)	31m 20s
check (ubuntu-20.04, cmake-build,ssl-vendored,gssapi-vendored,libz-static,curl-static,zstd, cmake...	5m 38s
	17h 38m 17s

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