chore: Update to fullcalendar@6

[larseggert]

Also properly apply bs5 styling to upcoming meeting calendar.

[codecov]

Codecov Report

Merging #5125 (de30640) into main (e280b25) will increase coverage by 0.01%.
The diff coverage is 75.30%.

@@            Coverage Diff             @@
##             main    #5125      +/-   ##
==========================================
+ Coverage   88.50%   88.51%   +0.01%     
==========================================
  Files         295      295              
  Lines       40015    40006       -9     
==========================================
- Hits        35415    35413       -2     
+ Misses       4600     4593       -7     

Impacted Files	Coverage Δ
ietf/doc/expire.py	95.31% <ø> (ø)
ietf/doc/views_ballot.py	92.86% <ø> (ø)
ietf/idindex/index.py	95.05% <ø> (ø)
ietf/ietfauth/utils.py	92.85% <ø> (ø)
ietf/ipr/utils.py	53.96% <0.00%> (ø)
ietf/meeting/views.py	90.70% <ø> (ø)
ietf/review/models.py	91.91% <ø> (ø)
ietf/stats/views.py	93.10% <ø> (ø)
ietf/utils/draft.py	71.44% <0.00%> (-0.31%)	⬇️
ietf/doc/views_draft.py	91.94% <25.00%> (ø)
... and 29 more

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