Update dev-lang/go-bootstrap to 1.18.6, by syncing with Gentoo commit
1b3154a4473deedaf3d1deb978f9a19739ac03af.
This is necessary because Go >= 1.20 requires go-bootstrap to be at
least 1.17.13.
Drop unnecessary Flatcar patches from go-bootstrap, and move the ebuild
from coreos-overlay to portage-stable.