Upgrade poise current to include poise next and serenity-next #914

Job	Run time
Build docs	1m 27s
Clippy	50s
Rustfmt	15s
Test (MSRV)	4m 1s
Test (stable)	2m 3s
Test (beta)	1m 59s
Test (nightly)	2m 2s
Test (macOS)	2m 45s
Test (Windows)	4m 32s
Test (no features)	1m 15s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 21s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 26s
	23m 56s

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