Update release-notes-8.16.asciidoc with known issue

[jlind23]

Add elastic agent uninstall on windows as a known issue to the 8.16 release note

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[kilfoyle]

Thanks for opening this @jlind23!
I have a similar PR open that also includes the "No agent found" issue. I thought I'd keep them in one PR to avoid a merge conflict. Also, that PR is in draft while we wait for some workaround suggestions.

Would it be okay if we use mine?

[jlind23]

Would it be okay if we use mine?

Absolutely.

[jlind23]

Moving this over to #1483