[6.x] Remove site content when a site is deleted

[ryanmitchell]

Fixes #11796

Sites::save() fires SiteDeleted per removed site but nothing cleaned up that site's content, so orphaned entries/trees/localizations remained and CollectionsController::getAuthorizedSitesForCollection() threw SiteNotFoundException on the stale handle still listed in each collection's sites config. This may have been intentional in the initial implementation but it's causing real world issues.

Fixed with a SiteDeleted subscriber that cascades cleanup synchronously.
Taxonomies/Term.php has 2 new methods - removeLocalization() + hasLocalization()

Behaviour on site deletion:

• Collections — detachLocalizations() re-roots surviving-site localizations, then the site's entries and its collection tree are deleted, and the site is removed from Collection::sites().
• Taxonomies — site removed from Taxonomy::sites(); each term loses that locale's data; a term with no localizations left is deleted; if the removed site was a term's root locale, a surviving localization is promoted so the term file stays valid.
• Navigations — the per-site nav tree is deleted (Nav::sites() is derived from trees).
• Globals — site + any origins pointing at it are removed from the set's sites map; the orphaned localization is dropped on save.

Any origin-linked content is flattened when its origin site is removed.

Known limitations:

• entry/term revision history isn't purged (matches Entry::delete() today);
• a chained origin (A←B←C) flattens to independent roots rather than re-parenting onto the surviving root (let me know if you'd prefer this to re-parent.
• statamic/eloquent-driver subclasses Term and will need its own removeLocalization for DB-backed term rows if/when this merges.