only consider versioned kernel packages in autoremove

Metapackages like "linux-image-amd64" are otherwise matched by our
extraction as well, which later on can't be successfully compared via
dpkg --compare-versions as the 'amd64' bit isn't a version number.
(Luckily none of our architectures starts with a digit.)

This was broken by me in 0.9.16 as I moved a shell-glob matcher to a
regex-based one which has slightly different semantics regarding '*'.

Closes: 741962