feat: use new meeting.Registration model

[rpcross]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 88.48485% with 19 lines in your changes missing coverage. Please review.

Project coverage is 88.91%. Comparing base (7c1479b) to head (047de24).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
ietf/meeting/utils.py	88.09%	15 Missing ⚠️
ietf/api/views.py	75.00%	3 Missing ⚠️
ietf/meeting/models.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8983      +/-   ##
==========================================
+ Coverage   88.79%   88.91%   +0.11%     
==========================================
  Files         314      314              
  Lines       41402    41468      +66     
==========================================
+ Hits        36764    36871     +107     
+ Misses       4638     4597      -41     

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