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

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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.