Prevent future dates in date of birth picker

[HeyangQin]

Summary

• Add maxValue extension with cqf-calculatedValue of today() to the birthDate field in the patient registration questionnaire
• The date picker already supports maxValue constraints via DatePickerViewHolderFactory.getSelectableDates() — the questionnaire simply didn't include one
• No code changes needed — the fix is purely in the questionnaire JSON definition, following the standard FHIR extension pattern

Test plan

• All MaxValueValidatorTest tests pass — confirms cqf-calculatedValue with date constraints works correctly
• All datacapture unit tests pass
• Manual: open demo app, tap '+' to add patient, verify date picker does not allow selecting future dates for date of birth

Closes #835