feat: exclude unlinked learner records from enterprise-customer-members results

[katrinan029]

Description

Currently, we are returning records for learners that have been unlinked from an enterprise but we only want to show learners that are linked. This PR updates the enterprise-customer-members endpoint to return results for linked learners of an enterprise.
https://2u-internal.atlassian.net/browse/ENT-9911

Testing instructions

1. link a user (http://localhost:18000/admin/auth/user/) to an enterprise customer via the manage users page. (I.e. http://localhost:18000/admin/enterprise/enterprisecustomer/7dbf461e-8d3d-4a4a-9b20-9c9121f04806/manage_learners).
   
2. visit the enterprise-customer-members endpoint and verify that you see the linked learner i.e. http://localhost:18000/enterprise/api/v1/enterprise-customer-members/7dbf461e-8d3d-4a4a-9b20-9c9121f04806
   
3. unlink the user from the enterprise http://localhost:18000/admin/enterprise/enterprisecustomer/7dbf461e-8d3d-4a4a-9b20-9c9121f04806/manage_learners
   
4. visit the enterprise-customer-members endpoint and confirm that you do not see the unlinked learner
   

Merge checklist:

• Any new requirements are in the right place (do not manually modify the requirements/*.txt files)
  • base.in if needed in production but edx-platform doesn't install it
  • test-master.in if edx-platform pins it, with a matching version
  • make upgrade && make requirements have been run to regenerate requirements
• make static has been run to update webpack bundling if any static content was updated
• ./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.
  • This should be run from either a venv with all the lms/edx-enterprise requirements installed or if you checked out edx-enterprise into the src directory used by lms, you can run this command through an lms shell.
    • It would be ./manage.py lms makemigrations in the shell.
• Version bumped
• Changelog record added
• Translations updated (see docs/internationalization.rst but also this isn't blocking for merge atm)

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 edx-platform to upgrade dependencies (including edx-enterprise)
  • Trigger the 'Upgrade one Python dependency' action against master in edx-platform with new version number to generate version bump PR
  • This must be done after the version is visible in PyPi as make upgrade in edx-platform will look for the latest version in PyPi.
  • Note: the edx-enterprise constraint in edx-platform must also be bumped to the latest version in PyPi.