Document that older Android versions have unmitigatable security risks on our supported platforms page

[Hixie]

Subcomponent of https://github.com/flutter/flutter/issues/63559.

We should update our public documentation to mention that versions of Android older than Android 9 (aka Pie, SDK level 28) leave applications vulnerable to the StrandHogg attack, and then link to https://developer.android.com/privacy-and-security/risks/strandhogg.