Inform the user that the --merged-usr flag is getting ignored for buildd...
Inform the user that the --merged-usr flag is getting ignored for buildd chroots of bookworm and earlier.
This seems to still confuse users. In this case, it triggered an RC bug against sbuild, expecting that sbuild-createchroot would create a merged-/usr variant buildd chroot for bookworm because the --merged-usr
flag was provided.
Closes: #1079938