fix: properly update studio search index when tags are deleted

[mgwozdz-unicon]

(cherry picked from commit 8bcdd46)

Description

This PR in conjunction with the openedx-core PR (openedx/openedx-core#571) is the backend component for openedx/modular-learning#260. The openedx-core PR (openedx/openedx-core#571) ensures that when a tag and its descendants are deleted that CONTENT_OBJECT_ASSOCIATIONS_CHANGED events take place. This PR ensures that once those events happen, the Meilisearch index is properly updated to remove the deleted tags. Without this PR, those events were only leading to a partial update of the document, which was sufficient to update renamed tags but not sufficient to remove deleted tags from the search index.

Goal:
When a Course Author deletes a tag that is associated to Library content, they should be able to return to the Library page and not see those tags in the tag search filter.

Supporting information

Github Issue: openedx/modular-learning#260

Testing instructions

1. Go to Taxonomies and select a taxonomy
2. Add new tags to this taxonomy that you plan to delete. (I have been adding a subtag (e.g. Wind1) with 2 sibbling children (W1.1 and W1.2) for my testing.)
3. Go to a Library and create 2 new pieces of content.
4. For one piece of content, associate the parent subtag (e.g. Wind1) and click Save.
5. For the other piece of content, associate a child (e.g. W1.2) and click Save.
6. Click on the tag search filter button and note that Wind1 and W1.2 appear in the tag search filter.
7. Return to the taxonomy and delete the subtag Wind1.
8. It should disappear from the taxonomy and a toast should display indicating what was deleted.
9. Go back to the Library and refresh the page in the browser.
10. The pieces of content you previously associated those tags to should now display that 0 tags are associated to them.
11. Click on the tag search filter button and note that Wind1 and W1.2 no longer appear in the tag search filter.

Deadline

This PR missed the Verawood code cut, and without it, there is a bug in the delete tags functionality.

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