bump edition to 2021 and set msrv to 1.71.1 #262

Job	Run time
Test Individual Features NoStd (binary)	16s
Build (ubuntu-latest, x86_64-unknown-linux-gnu)	21s
Test Individual Features NoStd (json)	14s
Rustfmt (ubuntu-latest, x86_64-unknown-linux-gnu)	8s
Build (ubuntu-latest, x86_64-pc-windows-gnu)	38s
Test Individual Features (binary)	13s
Build (ubuntu-latest, wasm32-unknown-unknown)	10s
Test Individual Features (json)	19s
Build (macos-latest, x86_64-apple-darwin)	14s
Test (ubuntu-latest, x86_64-unknown-linux-gnu)	20s
Test Individual Features NoStd (ron)	16s
Test Individual Features (ron)	32s
Build (windows-latest, x86_64-pc-windows-msvc)	1m 32s
Test Individual Features NoStd (toml)	11s
Test Individual Features (toml)	5s
Test No Std (ubuntu-latest, x86_64-unknown-linux-gnu)	23s
	5m 52s

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