versionmacros: min-require the current stable version

versionmacros: min-require the current stable version

.. instead of the previous stable version.

This ensures that if we use GDK_DEPRECATED_IN_3_6, it will actually emit
a warning in GTK 3.5, and not wait with that until GTK 3.7. This is
particularly useful for stuff that gets deprecated right now. This code
should emit warnings right now, so we know what we're doing while
deprecating.