fix: remove branch.refresh_from_db from branch serializer

[joseph-sentry]

Purpose/Motivation

using refresh_from_db on a branch object causes a similar issue as trying to update a branch object and saving using the django orm due to the primary key of the django model only being part of the actual composite primary key of the postgres table

Links to relevant tickets

Relevant ticket

What does this PR do?

• remove call to branch.refresh_from_db
• replace by using value of branch head returned by get_or_update_branch_head

[codecov-staging]

Codecov Report

All modified and coverable lines are covered by tests ✅

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[codecov-qa]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (daacd21) 96.08% compared to head (8f48b36) 96.08%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #368      +/-   ##
==========================================
- Coverage   96.08%   96.08%   -0.01%     
==========================================
  Files         632      632              
  Lines       16410    16409       -1     
==========================================
- Hits        15767    15766       -1     
  Misses        643      643              

Flag	Coverage Δ
unit	96.08% <100.00%> (-0.01%)	⬇️
unit-latest-uploader	96.08% <100.00%> (-0.01%)	⬇️

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[codecov-public-qa]

Codecov Report

Merging #368 (8f48b36) into main (daacd21) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #368      +/-   ##
==========================================
- Coverage   96.08%   96.08%   -0.01%     
==========================================
  Files         632      632              
  Lines       16410    16409       -1     
==========================================
- Hits        15767    15766       -1     
  Misses        643      643              

Flag	Coverage Δ
unit	96.08% <100.00%> (-0.01%)	⬇️
unit-latest-uploader	96.08% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
api/public/v2/branch/serializers.py	100.00% <100.00%> (ø)