[DateRangePicker] Stop forcing initial focus on the first section

[LukasTy]

Feedback received:
After entering a date of "12/12/2024", clicking outside the input, and clicking the year, focus unexpectedly jumps to the month. This seems to be caused by this focusField call: https://github.com/mui/mui-x/blob/master/packages/x-date-pickers-pro/src/internals/hooks/useEnrichedRangePickerFieldProps.ts#L182 (by not passing a newSelectedSection, it defaults to 0). Instead of entirely omitting this call, a more conservative fix would be to simply (Inside focusField: callback in useFieldV6TextField.ts) just not call setSelectedSections(newSelectedSection) if newSelectedSection is undefined).

from https://mui.com/x/react-date-pickers/date-range-picker/#basic-usage

The suggested change has a visual change that the correct section would no longer be selected when navigating (tabbing) between inputs.

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