chore: migrate edition 2018 to 2021 #1540

Job	Run time
Check Style	8s
Docs	1m 24s
unstable features	32s
MSRV	4m 55s
linux / nightly	1m 18s
Android	41s
WASM	51s
linux / stable	1m 19s
linux / beta	50s
macOS / stable	1m 38s
windows / stable-x86_64-msvc	4m 24s
windows / stable-i686-msvc	5m 17s
windows / stable-x86_64-gnu	8m 48s
windows / stable-i686-gnu	10m 11s
feat.: default-tls disabled	50s
feat.: rustls-tls	1m 51s
feat.: rustls-tls-manual-roots	1m 40s
feat.: native-tls	51s
feat.: default-tls and rustls-tls	1m 43s
feat.: cookies	55s
feat.: blocking	1m 59s
feat.: gzip	1m 39s
feat.: brotli	1m 26s
feat.: deflate	1m 46s
feat.: json	45s
feat.: multipart	1m 38s
feat.: stream	1m 20s
feat.: socks/default-tls	57s
feat.: socks/rustls-tls	1m 33s
feat.: trust-dns	1m 30s
CI is green	2s
	1h 4m 41s

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