[CAL-5131] Followup: 'Slot not longer available' message

[hariombalhara]

There is still a window where the user might be able to click the confirm button despite crossing to the minimumBookingNotice window, depending on the polling time (refetchInterval) and when they landed on the booking form page.

I think since we already know about the minimumBookingNotice, and it's a static information, we could in theory use that to disable the confirm button at precisely the right time, and that could be a nice follow up strategy to cover these cases.

Originally posted by @alishaz-polymath in #18935 (review)

Implement this and other followups in the PR.

_CAL-5131