fix: deal with "GMT" time zone in iCalendar files

[jennifer-richards]

A couple dozen interim meetings have been specified as the "GMT" timezone instead of "UTC". This is not handled correctly by the iCalendar renderer. This treats them as UTC. It also avoids a gross iCalendar syntax error if a timezone does not have a vtimezone file, though the resulting output will not comply with RFC 5545.

Fixes #8498

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.88%. Comparing base (e33a1a8) to head (3740bf3).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8506      +/-   ##
==========================================
+ Coverage   88.81%   88.88%   +0.06%     
==========================================
  Files         312      312              
  Lines       40856    40891      +35     
==========================================
+ Hits        36288    36347      +59     
+ Misses       4568     4544      -24     

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