chromium: Mark as insecure
Since M81 won't receive any updates anymore and there are known
vulnerabilities we should mark it as insecure so that users are aware of
the risks.
Updating Chromium to M83 is unfortunately too challenging for
19.09, but as of today we've already covered the one month period of
security updates for "oldstable" and both 20.03 and nixos-unstable
contain recent versions (i.e. users should either update to the current
stable release or install Chromium from a different channel).
nixos-unstable PR for M83: #88206