remove doc_auto_cfg, update MSRV #175

Job	Run time
Build and test (ubuntu-latest, stable)	44s
Miri (nightly)	1m 7s
Build no_std for thumbv7m-none-eabi	23s
Build and test (ubuntu-latest, 1.64)	5m 17s
Build no_std for thumbv6m-none-eabi	23s
Lint	35s
	8m 29s

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