Consolidate Dropdown components

[emteknetnz]

silverstripe/silverstripe-admin#1618 added SearchableDropdownFieldTrait which uses react-select to render a flexible drop down which can:

• be a regular drop down
• be a multi-select drop down
• type in to search and narrow down options
• optionally do lazy-loaded ajax requests for large amounts of options

In a CMS context, there are too many dropdown classes, including ones using jQuery chosen which we should be aiming to remove these and only use the react-select version

We will need to consider what impact this would have on projects that use the provided Form fields on the frontend - https://www.silverstripe.org/learn/lessons/v4/introduction-to-frontend-forms-1?_ga=2.202441553.1747665087.1701218520-1653549433.1678675180 as react isn't suitable for this

Since we can make breaking changes for major releases, SearchableDropdownFieldTrait.php should be turned into simply DropdownField.php and the following php files (and other associated files) should be removed:

• SelectField.php
• SingleSelectField.php
• DropdownField.php
• MultiSelectField.php
• ListboxField.php
• SearchableDropdownField.php
• SearchableMultiDropdownField.php
• SearchableDropdownTrait.php