chore: migrate edition 2018 to 2021 #1653

Job	Run time
Check Style	3s
Docs	1m 31s
linux / nightly	1m 13s
MSRV	4m 46s
Android	17s
unstable features	23s
linux / stable	1s
WASM	51s
linux / beta	1s
macOS / stable	1s
windows / stable-x86_64-msvc	1s
windows / stable-i686-msvc	2s
windows / stable-x86_64-gnu	1s
windows / stable-i686-gnu	1s
feat.: default-tls disabled	28s
feat.: rustls-tls	31s
feat.: rustls-tls-manual-roots	33s
feat.: native-tls	28s
feat.: default-tls and rustls-tls	31s
feat.: cookies	45s
feat.: blocking	32s
feat.: gzip	42s
feat.: brotli	28s
feat.: deflate	29s
feat.: json	30s
feat.: multipart	33s
feat.: stream	11s
feat.: socks/default-tls	35s
feat.: socks/rustls-tls	41s
feat.: trust-dns	1s
CI is green	0s
	17m 10s

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