configure: Always default to Qt6. * configure.ac: Remove condition that set the default version of Qt to Qt5 when building with a release tarball of Octave. I.e., the default is now Qt6 unconditionally. It can still be overridden with `--enable-qt=5`.

#!/bin/bash
## bootstrap script
## Run this to generate the configure script

set -e      # halt if unhandled error
aclocal
autoconf    # generate configure script