Release v3.3.0-alpha6

All unknown-variants return None, but this only
happens in practice with an empty variant (e.g.
a configuration key that is empty in the YAML).

FIXES #2237

[partition] make the tests also build with Qt6

This would lead to a runtime error when loading the QML.

Correct the comment description of the "Replace" partition behavior

Improve string formatting and context

FIXES #2018

Copy-paste-o, was using wrong string value for minimum size,
found by failing unit test.

While this isn't useful in production, it makes tests a lot
simpler to write since you don't have to wrestle with
creating new GS objects in order to start fresh.

While here, remove the textual entry in GS, which was unused.

CI: Switch Fedora Qt 6 builds to Fedora Linux 40

Fedora Linux 40 has the necessary components as part of upgrading
to KDE Plasma 6.