Upgrade poise current to include poise next and serenity-next #923

Job	Run time
Rustfmt	11s
Clippy	52s
Build docs	1m 21s
Test (MSRV)	1m 24s
Test (stable)	2m 8s
Test (beta)	1m 58s
Test (nightly)	2m 2s
Test (macOS)	3m 34s
Test (Windows)	4m 33s
Test (no features)	1m 20s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 19s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 26s
	22m 8s

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