Ensure datetimepicker widget overlay shows over modals & drop-downs

[lb-]

• Update to the exact same z-index as the modal (intentionally, as the widget will appear in the DOM after modals and sho show visually above the modals without needing to be an extreme z-index value).
• Fixes Wrong z-index of date/time/datetime pickers #3604 & fixed while working on Migrate datetime chooser init script to Stimulus w-date #10261
• Tested on Firefox 101, Chrome 111, Safari 16.1

Before

Note: I have toggled the datetime picker to show manually to make the problem more visible (dropdown AND datetimepicker shown at the same time.

After

[squash-labs]

Manage this branch in Squash

Test this branch here: https://lb-fix3604-datetimepicker-z-in-tjs1z.squash.io

[lb-]

Might be worth back-porting this fix to current LTS and maybe also 4.2 if merged.

[lb-]

Thanks Matt.