Add Support for site__in and site__not_in parameters

[burhandodhy]

Description of the Change

This PR adds the support for the site__in and site__not_in for the Post, Term and Comment Query. The old sites parameter will throw the deprecated warnings but keep working as it used to.

Closes #1781

How to test the Change

Changelog Entry

Added - site__in and site__not_in parameter support for Post, Term and Comment Query.
Deprecated - sites parameter is set to depreciated for. Post, Term and Comment Query.

Credits

Props @burhandodhy

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[felipeelia]

EXCELLENT work on the unit tests, @burhandodhy, thanks. I've left some comments here and there with some minor changes.

One thing we will need to address (and this became obvious with your new code) is that single-site and multisite tests are always run. It would be great if we could split those up and really only run the multisite tests in multisite.

In order to parallelize these tests executions, we could create a new workflow (simply duplicating this XML) and have single tests running in one and multisite tests running in the other. What do you think? Is that something you want to tackle in this PR?

In addition to that change, we will also want to exclude "Multisite.php" classes in single-site.xml.dist

[burhandodhy]

One thing we will need to address (and this became obvious with your new code) is that single-site and multisite tests are always run. It would be great if we could split those up and really only run the multisite tests in multisite.

In order to parallelize these tests executions, we could create a new workflow (simply duplicating this XML) and have single tests running in one and multisite tests running in the other. What do you think? Is that something you want to tackle in this PR?

In addition to that change, we will also want to exclude "Multisite.php" classes in single-site.xml.dist

I removed the multisite tests from the single-site.xml.dist. We already have another file phpunit.xml.dist which runs the all the tests including the multisite tests. Do we still need a separate file for only multisite tests?