Remove most version specifiers in `README` #600

Job	Run time
Pass MSRV values to other jobs	3s
nostd/thumbv6m-none-eabi/stable	15s
format/nightly	14s
docs/nightly	24s
clippy/stable	24s
miri	10m 59s
cargo-careful	1m 41s
cross_test/s390x-unknown-linux-gnu/stable	7m 51s
cross_test/i686-unknown-linux-gnu/stable	2m 7s
blas-msrv	54s
tests/beta	2m 19s
tests/stable	2m 22s
tests/nightly	2m 42s
tests/1.64.0	3m 25s
conclusion	4s
	35m 44s

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