Change references of `env_logger` to `tracing-subscriber` (#329) #1222

Job	Run time
Clippy	52s
Build docs	2m 16s
Rustfmt	7s
Test (MSRV)	1m 27s
Test (stable)	1m 52s
Test (beta)	2m 7s
Test (nightly)	1m 47s
Test (macOS)	1m 49s
Test (Windows)	5m 25s
Test (no features)	1m 6s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 14s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 24s
	21m 26s

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