FE: Use group-agent mapping API in Create Workspace UI

MR: Use group-agent mapping API to get available ag... (!148419 - merged)

Description

As a developer, I want to see the available agents where I can create a workspace in the "Create Workspace" page.

Acceptance Criteria

• This UI is already implements and we just need to change the underlying GraphQL query to obtain available agents.
• Ensure that the existing query and behavior is preserved if the feature flag is not enabled.
• Work with QA to ensure that the E2E / QA / Feature tests are updated, and ensure that a proper mapping record is created so the tests still work.

Technical Requirements

Use the new GraphQL query as described in the mapped cluster agents technical design document https://gitlab.com/gitlab-org/remote-development/gitlab-remote-development-docs/-/blob/main/doc/tech-designs/2024-01-23-support-group-agent-authorization.md?ref_type=heads