Clarify deprecation implications

[shainaraskas]

Summary

• finishes fixing [docs] Explain that the /_migration/deprecations API explicitly only affects the next major version elasticsearch#143234
• Inspired by Expand info about deprecation API usage  elasticsearch-specification#6070

Preview

https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/5415/deploy-manage/upgrade/prepare-to-upgrade/upgrade-assistant#_deprecations

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🔍 Preview links for changed docs

• deploy-manage/upgrade/prepare-to-upgrade/upgrade-assistant.md

[kilfoyle]

LGTM! ⛵

[DaveCTurner]

The {{es}} deprecation info API reports cluster, node, and index level settings that will be removed or changed in the next major version.

I think I'd rather we said that it reports on something more abstract like "features" and then use "cluster, node, and index level settings" as an example of those features. It's not just about settings, even if in practice today that's most of what this API handles.

[DaveCTurner]

LGTM