Deprecate `encoding::gamma` and its content #386

Job	Run time
Compile and test max MSRV (ubuntu-latest)	7m 16s
Compile and test min MSRV (ubuntu-latest)	2m 42s
Compile and test max MSRV (macos-latest)	5m 11s
Compile and test min MSRV (macos-latest)	2m 17s
Compile and test max MSRV (windows-latest)	10m 41s
Compile and test min MSRV (windows-latest)	3m 56s
Quick check (beta)	1m 41s
Miri tests	5m 31s
Quick check (stable)	1m 37s
Documentation	27s
integration tests (ubuntu-latest)	29s
Quick check (nightly)	1m 40s
integration tests (macos-latest)	32s
Test #[no_std]	17s
integration tests (windows-latest)	2m 34s
ci	1s
	46m 52s

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