Sitemaps: Rename the `wp_sitemaps_is_enabled` filter to `wp_sitemaps_enabled`.

This makes it more consistent with the other filters of the similar purpose.

Props arpitgshah.
Fixes #50428. See #50117.
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git-svn-id: https://core.svn.wordpress.org/trunk@47861 1a063a9b-81f0-0310-95a4-ce76da25c4cd