chore: migrate edition 2018 to 2021 #1598

Job	Run time
Check Style	11s
Docs	1m 37s
unstable features	34s
linux / nightly	1m 17s
MSRV	1m 57s
Android	23s
WASM	54s
linux / stable	1s
linux / beta	1s
macOS / stable	1s
windows / stable-x86_64-msvc	1s
windows / stable-i686-msvc	1s
windows / stable-x86_64-gnu	1s
windows / stable-i686-gnu	1s
feat.: default-tls disabled	1m 0s
feat.: rustls-tls	1m 12s
feat.: rustls-tls-manual-roots	1m 13s
feat.: native-tls	1m 4s
feat.: default-tls and rustls-tls	1m 11s
feat.: cookies	1m 12s
feat.: blocking	1m 16s
feat.: gzip	1m 2s
feat.: brotli	1m 5s
feat.: deflate	1m 6s
feat.: json	1m 3s
feat.: multipart	1m 12s
feat.: stream	38s
feat.: socks/default-tls	50s
feat.: socks/rustls-tls	42s
feat.: trust-dns	4s
CI is green	0s
	22m 50s

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