docs: note Kibana 7→8 migration known issue on 3.21 and 3.22 release notes

[tianfeng92]

Summary

• Add an upgrade note at the top of the 3.21.7 and 3.22.3 release sections calling out the known Kibana 7→8 saved-object migration issue when upgrading from Calico Enterprise 3.20 or earlier.
• The fix already shipped in 3.21.7 and 3.22.3 — the note points readers directly at those (or later) releases so the issue is avoided.

Test plan

• Verify the rendered note appears at the top of the 3.21.7 section on the 3.21 release-notes page.
• Verify the rendered note appears at the top of the 3.22.3 section (after the existing GitOps warning) on the 3.22 release-notes page.

🤖 Generated with Claude Code

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[ctauchen]

@tianfeng92 Can you provide more information about this? This seems ultimately to be a bug fix statement, so I'm not sure why we need to add a special admonition.

[tianfeng92]

@tianfeng92 Can you provide more information about this? This seems ultimately to be a bug fix statement, so I'm not sure why we need to add a special admonition.

@ctauchen The admonition is intentional because this is an upgrade-path hazard, not a typical runtime bug. Users upgrading from 3.20 or earlier will hit a Kibana crashloop during the saved-object migration — by the time they see it, they're already stuck in a broken state. The note is placed at the top of the release section so it's visible before someone starts the upgrade, rather than buried in the bug-fix list where it reads like a post-facto fix. Once 3.21.8 / 3.22.5 ship with the fix included, we can remove the admonition and leave just the bug-fix line item.