chore: migrate edition 2018 to 2021 #1541

Job	Run time
Docs	1m 32s
Check Style	7s
linux / stable	59s
linux / beta	41s
unstable features	44s
macOS / stable	1m 3s
linux / nightly	1m 57s
Android	47s
windows / stable-x86_64-msvc	42s
windows / stable-i686-msvc	41s
MSRV	3m 42s
windows / stable-x86_64-gnu	48s
WASM	42s
windows / stable-i686-gnu	50s
feat.: default-tls disabled	34s
feat.: rustls-tls	2m 59s
feat.: rustls-tls-manual-roots	1m 7s
feat.: native-tls	49s
feat.: default-tls and rustls-tls	47s
feat.: cookies	1m 37s
feat.: blocking	1m 36s
feat.: gzip	1m 33s
feat.: brotli	1m 5s
feat.: deflate	48s
feat.: json	3m 10s
feat.: multipart	50s
feat.: stream	1m 0s
feat.: socks/default-tls	47s
feat.: socks/rustls-tls	1m 43s
feat.: trust-dns	59s
CI is green	0s
	36m 39s

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