Test minimal dependency versions in CI #30

Job	Run time
Rustdoc	51s
Clippy	28s
Minimal Versions	1m 42s
Rustdoc with `cfg(docsrs)`	46s
Build	22s
Clippy (`tls-rustls`)	44s
Minimal Versions (`tls-rustls`)	1m 36s
Build (`tls-rustls`)	39s
Clippy (`tls-rustls-no-provider`)	29s
Minimal Versions (`tls-rustls-no-provider`)	1m 36s
Rustfmt	10s
Build (`tls-rustls-no-provider`)	18s
Clippy (`tls-openssl`)	34s
Minimal Versions (`tls-openssl`)	4s
Clippy (`--all-features`)	47s
Build (`tls-openssl`)	30s
Minimal Versions (`--all-features`)	14s
Build (`--all-features`)	51s
Test	33s
Build MSRV	1m 41s
Test (`tls-rustls`)	51s
Build MSRV (`tls-rustls`)	1m 38s
Test (`rustls/ring`)	38s
Build MSRV (`tls-rustls-no-provider`)	1m 41s
Test (`tls-openssl`)	39s
Build MSRV (`tls-openssl`)	1m 40s
Test (`--all-features`)	57s
Build MSRV (`--all-features`)	1m 41s
	24m 40s

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