Clarify Enterprise subscription requirement for Kibana fleet space-awareness

[herrBez]

Summary

Added note about Enterprise subscription requirement for space-aware data model.

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• deploy-manage/manage-spaces-fleet.md

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[wajihaparvez]

Thanks for updating the docs, @herrBez! I left a suggestion to make your addition more prominent.

I noticed that this page applies to both Serverless and Stack, but the subscription page linked in this PR is only for self-managed subscriptions. Are the subscription requirements for this feature the same across Serverless and Stack?

[herrBez]

In Serverless there is only Enterprise in my understanding 🤔 , so effectively the limitation is just for on-prem 🤔

[wajihaparvez]

@herrBez Makes sense. Two things to confirm:

• Is there no prerequisite to use this feature in Serverless?
• Does this feature apply to only self-managed/on prem? Or does it also apply to ECH/ECE/ECK? This will affect the link we use