Add known issue for Fleet setup > 1000 policies

[kilfoyle]

This updates the 8.12.0 Release Notes with a Fleet setup known issue, as requested here.

Josh, Kyle, let me know if I've misunderstood the issue.

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @kilfoyle? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[kpollich]

Thanks for adding this!

[gergoabraham]

@kilfoyle thank you for the documentation! 🙇

I'd like to suggest to change the number: actually it can happen above 500 agent policies (512 precisely)

[karenzone]

LGTM