Update to serenity 0.12.3 #1171

Job	Run time
Build docs	2m 37s
Rustfmt	6s
Test (MSRV)	1m 24s
Clippy	1m 6s
Test (stable)	2m 14s
Test (beta)	2m 4s
Test (nightly)	1m 48s
Test (macOS)	2m 15s
Test (Windows)	4m 55s
Test (no features)	1m 10s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 12s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 26s
	22m 17s

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