Fix DTSTART date when exporting `diary-float'

Fix DTSTART date when exporting `diary-float'

Instead of using current date as DTSTART and then excluding it
when it doesn't match the `diary-float' rule, just use the first
date which matches the rule in `icalendar-recurring-start-year'.
* lisp/calendar/icalendar.el (icalendar--convert-float-to-ical):
Compute the correct date.
* test/lisp/calendar/icalendar-tests.el: Add a test for the
reported test case.  (Bug#78085)

(cherry picked from commit c1153963b5db351d3fbe8f661e17efb55ebb2cb8)