Update & unpin serenity (#218) #899

Job	Run time
Build docs	4m 54s
Clippy	1m 6s
Rustfmt	10s
Test (MSRV)	1m 22s
Test (stable)	1m 59s
Test (beta)	2m 0s
Test (nightly)	1m 59s
Test (macOS)	3m 13s
Test (Windows)	5m 8s
Test (no features)	1m 47s
Test (all features + simd_json, cargo test --all-features --features serenity/simd_json --lib --t...	1m 23s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 23s
	26m 24s

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