Move group.cluster_agents field to CE

What does this MR do and why?

Describe in detail what merge request is being backported and why

This MR fixes the bug introduced in %17.10 when adding a group-level view for Kubernetes agents in !182628 (merged). The issue occurred because the group.clusterAgents field was added to the getAgents query, but this field was EE-only as it was added before the agent feature was moved to GitLab Core and wasn't being used then. This bug prevents users from seeing the agents list or creating an agent in the UI (both at project and group levels).

This MR resolves the issue by moving the group.clusterAgents field to CE.

Related MR for master branch: !186395 (merged)

MR acceptance checklist

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

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

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