Upgrade poise current to include poise next and serenity-next (#224) #927

Job	Run time
Build docs	2m 26s
Clippy	52s
Rustfmt	15s
Test (MSRV)	1m 24s
Test (stable)	2m 0s
Test (beta)	2m 1s
Test (nightly)	1m 59s
Test (macOS)	4m 41s
Test (Windows)	4m 1s
Test (no features)	1m 18s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 24s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 25s
	23m 46s

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