Update `CreatePoll` to use `u16` for duration #6382

Job	Run time
Examples	4m 10s
Test (macOS)	2m 2s
MSRV	1m 27s
Test (nightly)	2m 11s
Check minimal versions	1m 4s
Build docs	1m 3s
Test (native-tls)	1m 57s
Test (simd-json)	2m 19s
Test (Windows)	5m 41s
Test (no default features)	56s
Test (beta)	2m 29s
Test (unstable Discord API features)	2m 8s
Test (no cache)	2m 3s
Test (no gateway)	1m 32s
Test (builder without model, builder, true)	43s
Test (stable)	2m 8s
Test (unstable Discord API (no default features), unstable_discord_api, true)	46s
Test (unstable API + typesize, default unstable_discord_api typesize, true)	1m 21s
Test (chrono, chrono)	52s
	36m 52s

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