fix: BCPs can normatively cite all other standards levels

[larseggert]

Fixes #6524

[codecov]

Codecov Report

Merging #6530 (089690b) into main (f6a2d8c) will increase coverage by 0.15%.
Report is 40 commits behind head on main.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##             main    #6530      +/-   ##
==========================================
+ Coverage   88.70%   88.86%   +0.15%     
==========================================
  Files         290      284       -6     
  Lines       40440    40253     -187     
==========================================
- Hits        35874    35770     -104     
+ Misses       4566     4483      -83     

Files	Coverage Δ
ietf/doc/views_conflict_review.py	97.05% <100.00%> (-0.07%)	⬇️
ietf/secr/meetings/forms.py	89.33% <ø> (+0.16%)	⬆️
ietf/secr/meetings/urls.py	100.00% <ø> (ø)
ietf/secr/meetings/views.py	83.57% <100.00%> (+0.05%)	⬆️
ietf/secr/urls.py	100.00% <ø> (ø)
ietf/submit/parsers/base.py	96.72% <100.00%> (+1.56%)	⬆️
ietf/submit/parsers/xml_parser.py	100.00% <ø> (ø)
ietf/utils/xmldraft.py	93.18% <100.00%> (+0.26%)	⬆️
ietf/submit/forms.py	79.10% <80.00%> (+0.07%)	⬆️
ietf/doc/models.py	89.20% <90.47%> (+0.22%)	⬆️
... and 1 more

... and 4 files with indirect coverage changes