gtk-builder-tool simplify --3to4 now produces a
<requires lib="gtk" version="4.0"/>.
Adapt for that, and also add some tests that check that we
actually verify the requires, and accept 3.99 and 4.0, but
not 5.x.
This is rarely what you want, so lets turn it off
by default.
Update the one place in our demos where we want to
draw a value, add support for this to gtk-builder-tool,
add a test and mention this change in the migration
guide.
