Deprecate wp_get_sites()

[jeremyfelt]

Now that get_sites() has been introduced (See #35791) as part of WP_Site_Query, we can deprecate wp_get_sites(), which is not used within core at all.

This will entail:

• Add deprecation notice.
• Move function to ms-deprecated.php
• Either adjust all tests to use get_sites() or add @ExpectedDeprecated docs where appropriate. It may be good to leave all or some of the wp_get_sites() tests to help maintain backward compatibility.

[flixos90]

36994.diff​ deprecates wp_get_sites(). I annotated all tests that use wp_get_sites() with deprecated docs, since I think we should all keep them use the old wp_get_sites() - we already have tests for get_sites() in place, through the WP_Site_Query tests.

[flixos90]

I just noticed that another deprecated function get_blog_list() recommends wp_get_sites() as a replacement. Should this be adjusted as well to recommend get_sites() instead?

[jeremyfelt]

In 37653:

Multisite: Deprecate wp_get_sites()

Defer to the new get_sites() replacement, offering fresh (...or cached) WP_Site objects via the new WP_Site_Query.

Props flixos90.
Fixes #36994.