Changes the way we decide which content types are displayed in the settings.

[thijsoo]

Context

• We only want to show content types that are actually being indexed to show up in the settings.
  I added the same check for both taxonomies and custom post types because in both cases we sometimes don't create indexable. For example if they are not set public or excluded via one of our filters. I these cases we also do not want to display them in the menu sidebar because it does not make sense to optimise a page that is not indexed in the first place.
  The check implemented is exactly the same as the check used to determine if a cpt or tax should be an indexable.

Summary

This PR can be summarized in the following changelog entry:

• Updates the content types to only show indexable ones.

Relevant technical choices:

• Added a new function to more cleanly get objects instead of adding a parameter and doing an if check.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Make sure elementor is enabled.

• Without this PR you should see a my templates content type in https://wordpress.test/wp-admin/admin.php?page=wpseo_page_settings#/site-features

• Checkout this PR. The My templates should be gone.

• Make sure all your public post types are still there.

• add a public taxonomy with a plugin like custom cpt and make sure it shows up in the Categories & tags section.

• Make the taxonomy non public and make sure it is no longer there.

Relevant test scenarios

• Changes should be tested with the browser console open
• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on different editors (Block/Classic/Elementor/other)
• Changes should be tested on different browsers
• Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

QA can test this PR by following these steps:

Impact check

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

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Other environments

• This PR also affects Shopify. I have added a changelog entry starting with [shopify-seo], added test instructions for Shopify and attached the Shopify label to this PR.

Documentation

• I have written documentation for this change.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unit tests 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.
• I have written this PR in accordance with my team's definition of done.

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label and noted the work hours.

Fixes #

[igorschoester]

CR && AC ✅

Only you forgot the \ prefixes for the global namespace functions. I'll commit those from the comments and merge!