Upgrade poise current to include poise next and serenity-next #922

Job	Run time
Clippy	52s
Rustfmt	17s
Build docs	1m 20s
Test (MSRV)	1m 20s
Test (stable)	2m 5s
Test (beta)	1m 58s
Test (nightly)	1m 51s
Test (macOS)	4m 9s
Test (Windows)	4m 33s
Test (no features)	1m 18s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 23s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 26s
	22m 32s

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