bug#39938: 28.0.50; icalendar.el does not understand "first friday of every month"

[Mark A. Hershberger]

Version info:
    In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.5, 
cairo version 1.16.0)
     of 2020-03-04 built on silk
    Repository revision: cf45e8022ee182529668c0d50d27b4e168331e97
    Repository branch: master

Entry in my notes.org file contains the following note indicating the
event is the first friday of every month:

    ** Monthly meeting
    :PROPERTIES:
    … elided …
    :RRULE:    FREQ=MONTHLY;INTERVAL=1;BYDAY=1FR
    … elided …
    :END:

      <2019-07-05 11:30-12:30 +1m>


This matches the RRULE line I see when I “View source” on the event in
Zimbra.

Zimbra shows the date correctly as happening on Friday, 2020-03-06.

When I got my agenda for today (Thursday, 2020-03-05) from org-agenda,
it showed the event happening today.

If I use org-agenda-goto-date to find the the days that emacs thinks
this is on in other months, it always picks the 5th.

For example in May, the meeting is on 2020-05-05 (Tuesday) instead of
2020-05-01 (Friday).

This bug is similar to #39782 in that it deals with icalendar and
repeating days.