Forms: get rid of jQuery for date picker

[simison]

Get rid of jQuery requirement for Jetpack Forms date field; it's a big library to load just to show a simple date picker.

jetpack/projects/packages/forms/src/contact-form/js/grunion-frontend.js

Lines 1 to 10 in e8d1fd8

	jQuery( function ( $ ) {
	const $input = $( '.contact-form input.jp-contact-form-date' );
	const dateFormat = $input.attr( 'data-format' ) || 'yy-mm-dd';
	$input.datepicker( {
	dateFormat,
	constrainInput: false,
	showOptions: { direction: 'down' },
	} );
	} );

We can then update the library to something more modern and more accessible as well.

[github-actions]

This issue could use some more labels, to help prioritize and categorize our work. Could you please add at least a [Type], a [Feature], and a [Pri] label?

[simison]

We should try using native <input type="datetime-local" />: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/datetime-local