fix: Only consider ietf meetings as "next" for agenda_ical or agenda_json

[jennifer-richards]

Fixes #4629

[codecov]

Codecov Report

Merging #5238 (69d324d) into main (0ec1264) will decrease coverage by 0.02%.
The diff coverage is 92.45%.

❗ Current head 69d324d differs from pull request most recent head 3c6485f. Consider uploading reports for the commit 3c6485f to get more accurate results

@@            Coverage Diff             @@
##             main    #5238      +/-   ##
==========================================
- Coverage   88.58%   88.56%   -0.02%     
==========================================
  Files         295      294       -1     
  Lines       40114    40092      -22     
==========================================
- Hits        35534    35508      -26     
- Misses       4580     4584       +4     

Impacted Files	Coverage Δ
ietf/iesg/urls.py	100.00% <ø> (ø)
ietf/meeting/urls.py	81.25% <ø> (ø)
ietf/iesg/views.py	92.53% <85.18%> (-0.71%)	⬇️
ietf/doc/mails.py	96.20% <100.00%> (ø)
ietf/group/views.py	90.95% <100.00%> (+0.09%)	⬆️
ietf/meeting/helpers.py	89.84% <100.00%> (-0.07%)	⬇️
ietf/meeting/views.py	91.15% <100.00%> (+0.03%)	⬆️
ietf/secr/proceedings/proc_utils.py	86.33% <100.00%> (ø)
ietf/utils/text.py	85.97% <0.00%> (-1.22%)	⬇️
ietf/meeting/models.py	86.03% <0.00%> (-0.05%)	⬇️
... and 2 more

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