First pass at getting data for XML sitemaps from indexables

[jdevalk]

Summary

This PR can be summarized in the following changelog entry:

• XML sitemaps are now beeing fed from the Indexables database.

Relevant technical choices:

• Decided to remove the wpseo_sitemap_exclude_empty_terms and wpseo_sitemap_exclude_empty_terms_taxonomy filters. To remove terms from the XML sitemap, make sure they're set to noindex in Indexables.
• Removed the wpseo_hit_sitemap_index action.
• Removed the transient caching (which we defaulted not to use anyway so nobody had on probably).
• The wpseo_sitemap_entry filter has the object_id, so the WordPress ID for the object, as its third parameter now.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Every indexable post type URL should be in the XML sitemaps;
• Every indexable post type archive URL should be in the XML sitemaps;
• Every image inside a post should be in the XML sitemap;

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

• This has a HUGE impact. Please test thoroughly.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.

Fixes #

[diedexx]

@jdevalk this line should be removed, right? It makes it that the second most recent last modified date is shown for each page instead of the most recent. Or is there something I'm missing?

[diedexx]

Replaced with #17869