Fix prolearn bug for existing resources whose urls have changed

[mbertrand]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/5911

Description (What does it do?)

Handles situations where another field besides readable_id also needs to be unique (url for prolearn), and an existing resource with that readable_id exists with a different url than what the API returns.

How can this be tested?

• Start on the main branch.
• If you don't already have it, create a LearningResource with a readable_id of prolearn-mitpe-712:

  from learning_resources.models import *
  LearningResource.objects.update_or_create(
    readable_id="prolearn-mitpe-712",
    etl_source="prolearn",
    platform=LearningResourcePlatform.objects.get(code="mitpe"),
    offered_by=LearningResourceOfferor.objects.get(code="mitpe"),
    defaults = {
      "url": "https://professional.mit.edu/course-catalog/designing-efficient-deep-learning-systems-live-virtual"
    }
  )

• Run ./manage.py backpopulate_prolearn_data, it should fail with a duplicate key error.
• Switch to this branch, restart containers, and try again. It should complete successfully.
• Check that the resource with readable_id=prolearn-mitpe-712 has a new url of https://professional.mit.edu/course-catalog/designing-efficient-deep-learning-systems-live-online

[abeglova]

lgtm