'parent_item' taxonomy label should be used in tag edit form

[abrain]

When adding or editing hierarchical taxonomies, the label for the parent dropdown is always 'Parent'. This might be confusing for the user when dealing with custom taxonomies where the term 'Parent' is not the best fit. Instead, the taxonomy's 'parent_item' label should be used, as it is done in post_categories_meta_box().

[dlh]

34545.diff​ would implement this suggestion on edit-tags.php and edit-tag-form.php.

I would also note that when a taxonomy doesn't define the parent_item label, the default is used, which is currently Parent Category, and which might not be a good fit either.

So, an alternative approach could be to use a generic string (like Parent or Parent Term) everywhere.

[johnbillion]

Replying to dlh:

I would also note that when a taxonomy doesn't define the parent_item label, the default is used, which is currently Parent Category, and which might not be a good fit either.

All the labels fall back to their Category/Tag version, so this is fine as-is.

[boonebgorges]

In 40146:

Taxonomy: Improve 'Parent' label when editing taxonomy terms.

Instead of the hardcoded 'Parent', use the parent_item label.

Props abrain, dlh.
Fixes #34545.