remove: remove utils/capitalize-keys

This commit removes `@stdlib/utils/capitalize-keys` in favor of
`@stdlib/object/capitalize-keys`.

BREAKING CHANGE: remove `utils/capitalize-keys`

To migrate, users should update their require/import paths to use
`@stdlib/object/capitalize-keys` which provides the same API and implementation.

Ref: #8755

Author: Neerajpathak07