Upgrade poise current to include poise next and serenity-next #915

Job	Run time
Clippy	50s
Build docs	1m 19s
Test (MSRV)	3m 50s
Rustfmt	12s
Test (stable)	2m 3s
Test (beta)	2m 2s
Test (nightly)	1m 56s
Test (macOS)	4m 40s
Test (Windows)	4m 9s
Test (no features)	1m 15s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 17s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 20s
	24m 53s

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