refactor!: require fake timers for mocking date

[hi-ogawa]

Description

WIP

While checking #7329, I was slightly confused about this feature allowing mocking date vi.setSystemTime without vi.useFakeTimers. I'd assume historically Vitest had "mockdate" feature (vi.mockCurrentDate), which is later replaced with Jest-like API vi.setSystemTime #531, but then that required vi.useFakeTimers, so vi.setSystemTime only mocking is added back in #639.

Having two system internally (and two flags _fakingTime and _fakingDate) looks confusing and maybe removing this is fine later time on breaking change. Users can probably migrate to vi.useFakeTimers({ toFake: ["Date"] }).

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