[Fleet] Update references to subscription requirements

[vishaangelova]

This PR:

• Removes references to specific subscription levels in the Fleet docs.
• Clarifies the docs for subscription-based features by highlighting the name of the feature as it is listed on the Subscriptions page.

Closes #2780

[github-actions]

🔍 Preview links for changed docs

• reference/fleet/agent-policy.md
• reference/fleet/automatic-integrations-synchronization.md
• reference/fleet/data-streams-advanced-features.md
• reference/fleet/fleet-settings.md
• reference/fleet/index.md
• reference/fleet/integration-level-outputs.md
• reference/fleet/remote-elasticsearch-output.md
• reference/fleet/unenroll-elastic-agent.md
• reference/fleet/upgrade-elastic-agent.md

[karenzone]

Nice improvements! I like the way you seamlessly worked in the feature name that users should search for.

What do you think about changing the link text to "Elastic subscriptions?" Even though we're deep linking to the self-managed page, "Elastic Stack subscription" doesn't seem like something a user would say.

I left some minor comments inline for your consideration. Otherwise, LGTM! 🎉