scripts: gutsy: Allow generation of chroot for Ubuntu releases in ESM

This commit changed the logic and currently in questing (25.10) or resolute (26.04) it won't be possible to create chroots for Ubuntu releases in ESM, e.g. bionic (18.04). This was a quick fix to it, but the code still has another issue with the keyrings, as it is incorrectly assuming that a unsupported release key is in the ubuntu-archive-removed-keys.gpg and that is not true for all end-of-life releases, but this is right now less of a priority.