Add rust-version, so that our MSRV is enforced by cargo: #534

Job	Run time
lint	7m 45s
lint	7m 45s
check-minimal-versions	3m 58s
check-minimal-versions	3m 58s
test (7.5.1, 3.5)	6h 0m 16s
test (7.5.1, 3.5)	1h 5m 48s
check (macos-11.0)	8m 12s
check (macos-11.0)	8m 12s
test (5.3.1, 2.3)	19m 1s
test (5.3.1, 2.3)	19m 1s
check (windows-2019, cmake-build,libz-static,curl-static, cmake-build,libz-static,curl-static)	13m 22s
check (windows-2019, cmake-build,libz-static,curl-static, cmake-build,libz-static,curl-static)	13m 22s
test (5.0.3, 2)	16m 22s
test (5.0.3, 2)	16m 22s
check (ubuntu-20.04, tracing)	5m 0s
check (ubuntu-20.04, tracing)	5m 0s
test (4.1.3, 1.1)	1h 58m 23s
test (4.1.3, 1.1)	3h 9m 47s
check (ubuntu-20.04, cmake-build,ssl-vendored,gssapi-vendored,libz-static,curl-static,zstd, cmake...	5m 35s
check (ubuntu-20.04, cmake-build,ssl-vendored,gssapi-vendored,libz-static,curl-static,zstd, cmake...	5m 35s
	14h 52m 44s

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