[Development] Changes to Qt offering

[David Edmundson]

> All security fixes are made available to everyone, for all Qt versions that
> they affect, provided it's still a supported Qt version (or it was easy to
> make the fix).
>
If we could have that explicitly in writing from TQC, that would mean a lot.

I can easily envision a situation that affects only Qt5.15, but not
Qt6.0 at which point it's not covered by what has been suggested
officially so far as there would be nothing to cherry-pick.

David