Contact Form / Date Picker: Option to restrict dates

#### Is your feature request related to a problem? Please describe.
A user wants to be able to use the Date field on the form, but not allow users to pick dates that are in the past.

#### Describe the solution you'd like
An option to restrict dates to future only or past only would be nice.

#### Describe alternatives you've considered
I'm going to recommend the Calendly block as an alternative, since that may suit this person's needs.

#### Additional context
Forum link: https://wordpress.com/forums/topic/how-can-i-restrict-users-from-selecting-any-date-before-today-in-the-date-field-of-my-form/

No implementation files, tests, or entry points are named in the issue. Start by locating the Contact Form date field and date-picker code, then find its existing tests; done means users can be restricted to future-only or past-only dates and the behavior is covered by tests.

Written by the indexing model from the issue text.