Migrate approvers_select GLDropdown to <select>

What does this MR do and why?

For #424760 (closed)

Converts a GlDropdown to be a <select> on the approvers_select.vue component.

Per the Dropdown Combobox documentation, this dropdown qualifies to be a standard <select> element.

Screenshots or screen recordings

Before	After
	(My operating system is Ubuntu)

How to set up and validate locally

1. Have an enterprise version of the GDK (this is not a free feature)
2. Create a new or edit an existing MR
3. Open the "Approval rules" expansion section
4. Click "Add approval rule"
5. The dropdown is in the "Add approvers" section

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.