fix: cast wp_count_terms() result to int before using in ceil()

[justlevine]

Trac ticket: https://core.trac.wordpress.org/ticket/52217

This PR fixes two instances of the (numeric string) return value from wp_count_terms() being used directly in ceil(), which expects an int|float.

Affected methods:

• WP_Sitemaps_Taxonomies::get_max_num_pages()
• wp_nav_menu_item_taxonomy_meta_box()

While this issue was surfaced via PHPStan in #7619 (trac: https://core.trac.wordpress.org/ticket/61175 ), it can be remediated independently of that ticket.

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