feat: modify queryset from prefetch to select related

[uzairr]

PROD-3860

This PR modifies prefetch queryset to select_related query to reduce db round-trip for a skill fetching.

Testing:

• Clone taxonomy in edx/src directory.
• Install taxonomy via discovery shell in editable mode

pip install -e /edx/src/taxonomy-connector

• Comment cache-related code to verify the impact
• Add Course Skills to courses via discovery admin.
• Check query count on master and ensure the response has skills information.
• Checkout modify-queryset and hit the endpoint again. Note the decrease in query count.

---

• Tested search/all endpoint with 2 scenarios:
  Course with multiple skills -> (with no skill_category and skill_subcategory)
  • with select_related: 94-96 queries
  • with prefetch_related: 93 queries
    Courses with multiple skill -> (having skill_category and skill_subcategory)
  • with select_related: 106 queries.
  • with prefetch_related: 112 queries

Merge checklist:

• Any new requirements are in the right place (do not manually modify the requirements/*.txt files)
  • make upgrade && make requirements have been run to regenerate requirements
• ./manage.py makemigrations has been run
  • Checkout the Database Migration Confluence page for helpful tips on creating migrations.
  • Note: This must be run if you modified any models.
    • It may or may not make a migration depending on exactly what you modified, but it should still be run.
• Version bumped
• Changelog record added

Post merge:

• Tag pushed and a new version released
  • Note: Assets will be added automatically. You just need to provide a tag (should match your version number) and title and description.
• After versioned build finishes in GitHub Actions, verify version has been pushed to PyPI
  • Each step in the release build has a condition flag that checks if the rest of the steps are done and if so will deploy to PyPi.
    (so basically once your build finishes, after maybe a minute you should see the new version in PyPi automatically (on refresh))
• PR created in course-discovery to upgrade dependencies (including taxonomy-connector)
  • This must be done after the version is visible in PyPi as make upgrade in course-discovery will look for the latest version in PyPi.

[Ali-D-Akbar]

nit: prefetch related to select related for whitelisted product skills.