[8.16](backport #4656) Remove Enterprise subscription mention

[mergify]

Description

We no longer mention specific licenses in the docs and instead link users to the subscriptions page.
This PR adds the link to the subscriptions page and removes the mention of the Enterprise license in the prereqs.

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)

• Serverless (docs/en/serverless/*)

• Integrations Developer Guide (docs/en/integrations/*)

• None of the above

• Product/Engineering Review

• Writer Review

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This is an automatic backport of pull request #4656 done by [Mergify](https://mergify.com).

[github-actions]

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