[Admin] Improve dropdowns for new products

[drummer83]

Description

Follow up of #12760.

On page admin/products/new there are some dropdowns, which can be improved.

Expected Behavior

• Supplier:
  • String "Select a supplier" is not applied anymore. We can live with that, because we don't have it for other fields as well. But remove the string from en.yml then, if unused elsewhere?
  • String "Search for suppliers" is not applied as placeholder, the dropdown is not searchable. Could make sense to add the search functionality and placeholder in the search field.
    • EDIT: Just found out, that the dropdown IS searchable, if you simply start typing. It's poor UX though, because users can't know that.
  • The blank selection is not required, because the field is mandatory.
• Unit size:
  • No need to have this field searchable (short list only).
  • String "Search for units" is not applied as placeholder. Remove from en.yml?
• Product category:
  • The blank selection is not required, because the field is mandatory.

Steps to Reproduce

1. Visit /admin/products/new.

Animated Gif/Screenshot

Kazam_screencast_00020.webm