fix(DatePicker/DateRangePicker/Calendar/RangeCalendar)!: make weekStartsOn locale-independent

[kricsleo]

resolves #2157
resolves #2264

❗❗This may be a breaking change for those who rely on weekStartsOn to behave differently across locales.

As I've seen more users confused by this implicit behavior, I think it's better to define weekStartsOn in a fixed and explicit way, where the day of the week is always Sun(0), Mon(1), ..., Sat(6). This also aligns with other date libraries, e.g., date-fns, making it more predictable.

However, this would break users who rely on the previous behavior by specifying different weekStartsOn values for different locales to make the calendar always start on "Monday" or other days.

For example:

Before: locale="en-GB" :weekStartsOn="0" === locale="en-US" :weekStartsOn="1" === "week starts on Monday"

After: locale="en-GB" :weekStartsOn="1" === locale="en-US" :weekStartsOn="1" === "week starts on Monday"

I've also considered adding a new prop (e.g., fixedWeekStartsOn: boolean) to only change the behavior when this prop is set, which could avoid breaking changes. However, this would add complexity to the library, and the default behavior would still remain confusing (and we'd have to document it, etc.). WDYT? cc @zernonia

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commit: f162481