Bump Django to 4.2.13

[EvaBardou]

Depends on #7814

Closes #7794

Three code updates were needed for this bump, see Django 4.2 release notes for all changes:

• STATICFILES_STORAGE is deprecated and is to be replaced by STORAGES['staticfiles'] -> https://docs.djangoproject.com/en/4.2/releases/4.2/#custom-file-storages
• index_together is deprecated in favor of indexes -> https://docs.djangoproject.com/en/4.2/releases/4.2/#index-together-option-is-deprecated-in-favor-of-indexes
• It's recommended to set update_fields in custom Model.save() methods to ensure fields are properly saved -> https://docs.djangoproject.com/en/4.2/releases/4.2/#setting-update-fields-in-model-save-may-now-be-required

---

I've tested the bump locally by exporting:

export DATABASE_URL=psql://postgres:mozilla1234@postgres:5432/treeherder

to use the postgres database instead of the mysql one.

And running:

docker-compose up --build

[EvaBardou]

I need to submit a new PR removing mysql before we merge this one.

[EvaBardou]

@Archaeopteryx Since #7814 was just merged, I've rebased this PR on master, it is ready for review. 😄

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 90.36%. Comparing base (494ca3c) to head (6089a69).

Files	Patch %	Lines
treeherder/perf/models.py	70.00%	3 Missing ⚠️
treeherder/model/models.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #7811       +/-   ##
===========================================
+ Coverage   58.56%   90.36%   +31.79%     
===========================================
  Files         239      308       +69     
  Lines       11262    15719     +4457     
  Branches     3385        0     -3385     
===========================================
+ Hits         6596    14204     +7608     
+ Misses       4504     1515     -2989     
+ Partials      162        0      -162     

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