Add cleanup in DocumentUrl tables when the content key changes.

[KevinJump]

Workaround for https://github.com/umbraco/Umbraco-CMS/issues/21131.

When a content item's Key changes during an import - Umbraco throws an ForeignKey exception because of the bindings to the umbracoDocumentUrl table.

This PR checks on key changes, and removes entries from that table (via the repository). - so the conflict doesn't happen.

On Save Umbraco puts new entries in the table with the new Key so this "should" be ok.

[KevinJump]

[!WARNING] (Breaking - changes the Constructor for the ContentSerializer class)