Add discard_overstep function to Time<Fixed> #31991

Job	Run time
build (windows-latest)	4m 39s
check-bevy-internal-imports	5s
check-doc	8m 15s
build (ubuntu-latest)	3m 33s
check-missing-examples-in-docs	31s
build (macos-latest)	4m 47s
ci	3m 24s
miri	5m 58s
run-examples-on-windows-dx12	13m 17s
check-missing-features-in-docs	27s
markdownlint	1m 11s
check-compiles	2m 25s
msrv	1m 18s
build-wasm	46s
	50m 36s

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