Make date props not optional

[Moritz-Steinle]

Since the date prop is declared required in the docs, it should be in the code. Currently however it's the following: export interface CountdownProps extends React.Props<Countdown>, CountdownTimeDeltaFormatOptions, Omit<LegacyCountdownProps, 'onComplete'> { readonly date?: Date | number | string; [...]

[ndresx]

True, I must have changed that for legacy reasons, will check if this can be corrected! Thank you!