deprecate Member::distinct #5882

Job	Run time
Test (macOS)	1m 28s
Test (nightly)	3m 15s
Build docs	35s
Examples	3m 43s
Test (stable)	2m 59s
Check minimal versions	38s
Test (beta)	3m 9s
MSRV	44s
Test (Windows)	4m 57s
Test (native-tls)	2m 46s
Test (no default features)	40s
Test (no cache)	2m 51s
Test (no gateway)	1m 46s
Test (unstable Discord API features)	3m 10s
Test (simd-json)	3m 2s
Test (chrono, chrono)	49s
Test (unstable API + typesize, default unstable_discord_api typesize, true)	58s
Test (builder without model, builder, true)	25s
Test (unstable Discord API (no default features), unstable_discord_api, true)	26s
	38m 21s

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