BUG: only the "post" indexable is acted upon when a site's status changes

[dustinrue]

Describe the bug
When a site in a multisite setup is updated so that it is archived, deleted or marked as spam, only the "post" indexable will be acted upon because it is the only indexable that adds actions for those items (https://github.com/10up/ElasticPress/blob/3.6.1/includes/classes/Indexable/Post/SyncManager.php#L53 vs https://github.com/10up/ElasticPress/blob/3.6.1/includes/classes/Indexable/Term/SyncManager.php#L37). This leaves data on the backend server.

Steps to Reproduce

1. Have a multisite
2. Create an additional site
3. Enable some other indexable in the dashboard, such as terms
4. Index all sites
5. Note the indexes created in the backend
6. Mark a site as deleted
7. Note the index that still exist in the backend will include indexable items other than post

Expected behavior

All indexes should be removed from the backend server/service

Screenshots

Environment information

• Device:
• OS:
• Browser and version:
• WordPress version:

• ElasticPress version:
• Elasticsearch version:
• Where do you host your Elasticsearch server?
• What ElasticPress features do you have currently active?
• Plugins and version:
• Theme and version:

• Site Health Info:

Additional context

[felipeelia]

To have it fixed on all Indexables, we probably should move that code to the Indexable class, testing if it is global or not and acting accordingly.