Add not supported message to organization switcher
What does this MR do and why?
Related to #440741 (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/
In the first iteration of Organizations and Cells organization switching will not be supported. This MR introduces a new organization_switching feature flag that updates the UI when disabled. When that feature flag is disabled the organization switcher will say Switching between organizations is not currently supported. [Learn more.](https://docs.gitlab.com/ee/user/organization/). As we move towards supporting organization switching we will be able to enable this feature flag to to test.
MR acceptance checklist
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Screenshots or screen recordings
How to set up and validate locally
- Enable the feature flag
echo "Feature.enable(:ui_for_organizations)" | gdk rails c
- Open the organization switcher at the top of the sidebar