fix(a11y): hide clear button on required select and array widgets

[Wagner3UB]

Partially fixes #5140

What

Improves keyboard accessibility for the clear button and hides it on required fields in SelectWidget and ArrayWidget.

Changes

• SelectWidget / ArrayWidget: isClearable now respects the required prop — the clear button is hidden on required fields.
• SelectStyling — ClearIndicator: overrides react-select's default aria-hidden="true" on the clear button, adding role="button", tabIndex=0, and an onKeyDown handler so the button is reachable via Tab and activatable with Enter or Space.

Behavior after fix

• Required fields no longer show the clear button (e.g. "Portal default timezone", "First weekday" in the Date and Time control panel).
• Non-required fields: the clear button is now reachable via Tab and can be activated with Enter or Space.

[stevepiercy]

A minor tweak and a question.

[stevepiercy]

A sprinkling of MyST fair dust.

Suggested change

	Hide the clear button in `SelectWidget` and `ArrayWidget` when the field is marked as required, and make it keyboard accessible (Tab, Enter, Space) on non-required fields. @Wagner3UB
	Hide the clear button in `SelectWidget` and `ArrayWidget` when the field is marked as required, and make it keyboard accessible ({kbd}`Tab`, {kbd}`Enter`, {kbd}`Space`) on non-required fields. @Wagner3UB

[stevepiercy]

Just to verify the desired behavior, if there are multiple controls each with a clear indicator of tabindex=0, then first the controls would be tab-able in the order they appear, followed by the clear buttons in the order they appear.

If it were me, instead I'd like to have the clear button tab-able immediately after its control.