Improve cleanup of cached term_ids after shared terms are split.

* If the split term ID is stored as 'default_category', 'default_link_category', or 'default_email_category', update it to the new ID.
* If the split term ID is associated with a nav menu item, update that piece of postmeta to the new ID.

Props mboynes.
See #30335.
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git-svn-id: http://core.svn.wordpress.org/trunk@30483 1a063a9b-81f0-0310-95a4-ce76da25c4cd