Fix ordering of kernel version strings for multiple Debian revisions

Currently the ABI name for the second (or later) Debian version with the same upstream version and target release has a '+n' suffix. This sorts as intended in isolation, e.g.:

6.19.6+deb14+1 > 6.19.6+deb14

but once a flavour suffix is added the ordering is reversed:

6.19.6+deb14+1-amd64 < 6.19.6+deb14-amd64

('linux-version sort' and 'sort --version-sort' agree on this ordering. 'dpkg --compare-versions' orders them the way we want, but these are not package versions and it shouldn't be used for them.)

If we change the '+' separator to a letter then 'linux-version sort' will order the strings as we want, but 'sort --version-sort', used by GRUB, will not.

So change the '+' separator to '.'.

Closes: #1113728