feat: replace agenda_warning_note with "preliminary" flag

[jennifer-richards]

Description

The agenda_warning_note for a meeting is used to mark agendas as preliminary. The prominent display of the note was restored in #4124, but the way it is displayed there is not appropriate for free-form text.

The free-form field should be dropped in favor of a preliminary flag to indicate that the meeting's agenda is not yet final.

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