fix(wp5.9): taxonomy panel filter

[dkoo]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Looks like the props passed to the editor.PostTaxonomyType filter have changed in WP 5.9. It no longer passes a taxonomy prop containing info about the taxonomy, such as whether it's hierarchical or its human-readable labels. However, it does still pass the slug prop, which we can still use to determine whether the panel being rendered is for Categories or Tags. Relying on this slug prop alone should fix errors in WP 5.9 as well as maintain backwards compatibility with WP 5.8 and earlier.

Unrelated, but also adds the missing CHANGELOG.md file that will be needed for the new post-release CI job implemented in #141.

Fixes #148.

How to test the changes in this Pull Request:

1. On master, edit a sponsor and attempt to expand the Categories or Tags sidebar panels.
2. Observe an editor crash as described in "TypeError: taxonomy is undefined" error with WordPress 5.9 RC #148.
3. Check out this branch rebuild, refresh, and confirm that the crash no longer happens and that you can once again add/remove categories and tags using both sidebar panels.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[laurelfulford]

You may want to have a more technical set of eyes check over this as well, but in my testing this PR fixed the issue and allowed me to load the editor and add/edit categories and tags using the 5.9 RC 2.

I also rolled my test site back to 5.8.3, I could still edit a sponsor and change the categories and tags with no issue. So it all looks good from a functional perspective! :)

[dkoo]

The changes are pretty light from a logical perspective (it removes code instead of adding any), so I think we're good!

[matticbot]

🎉 This PR is included in version 1.8.0-alpha.1 🎉

The release is available on GitHub release

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