Use center placement for organization switcher

What does this MR do and why?

Related to #441158 (closed)

The grouptenant scale team is starting to build out an Organization MVC. For more information about what an "Organization" will be see https://docs.gitlab.com/ee/architecture/blueprints/organization/

This MR adds the placement="center" prop to GlDisclosureDropdown in the organization switcher to fix the placement.

MR acceptance checklist

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Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Open Rails console
   • bin/rails console
2. Enable the feature flag
   • Feature.enable(:ui_for_organizations)