feat!: remove effort field from CourseCatalogData

[rgraber]

Description:
Removes "effort" field in CourseCatalogData. This field is not actually editable in Studio in setups where Publisher is enabled. Also updates the ADR around COURSE_CATALOG_INFO_CHANGED event design to clarify the fields not being copied from the courses API endpoint.

Note that this is in fact a breaking change and will require some work in edx-platform (and possibly discovery) to bring it in.

Issue: #133

Reviewers:

• tag reviewer
• tag reviewer

Merge checklist:

• All reviewers approved
• CI build is green
• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Commits are squashed

Post merge:

• Create a tag
• Check new version is pushed to PyPI after tag-triggered build is
  finished.
• Delete working branch (if not needed anymore)

[robrap]

Thanks.

[rgraber]

Ended up rebasing/squashing to make sure my commit messages were all compliant, and because I forgot that would make your life slightly harder. I made all suggested changes to the ADR, changed the version to 3.0.0, and updated the Changelog. Those should be the only changes since your last review.