Disallow "GMT" as a time zone choice

[jennifer-richards]

Description

Our time zone list includes "GMT", at least for creating interim meetings (likely everywhere). While pedants might applaud the distinction, it should not be an option: "UTC" should be used instead.

The list could be filtered in the form, but we should explore how we're building up the ietf.meeting.models.timezones in the top of ietf/meeting/models.py. If we can safely filter out "GMT" there, we should.

This is relevant to #5635 and perhaps #5683.

See also #8506, which adjusts our iCalendar renderer to handle "GMT", and #5393, which tracks refactoring how we generate iCalendar files.

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