Site Health: Rename `WP_Site_Health::initialize()` introduced in [47063] to...

Site Health: Rename `WP_Site_Health::initialize()` introduced in [47063] to `::get_instance()`, for clarity and consistency with other core classes.

Use `WP_Site_Health::get_instance()` where it's needed, instead of creating multiple instances of the class.

Props afercia, xkon, Clorith, SergeyBiryukov.
See #47606.
Built from https://develop.svn.wordpress.org/trunk@47149


git-svn-id: https://core.svn.wordpress.org/trunk@46949 1a063a9b-81f0-0310-95a4-ce76da25c4cd