Add lts and only-latest-lts matrix build targets + drop 9p doublefree patch for > 6.18.15

[bleggett]

This adds only-latest-lts and lts to the existing spec targets, so we can lock CI to the former.

Ex:

~/Source/edera-dev/linux-kernel-oci » hack/build/generate-matrix.sh only-latest-lts
generated 1 builds
build zone 6.18.16 for x86_64 with tags 6, 6.18, 6.18.16 to ttl.sh/ben-ftrace/zo
ne-kernel, ttl.sh/ben-ftrace/zone-kernel-sdk on ubuntu-latest

~/Source/edera-dev/linux-kernel-oci » hack/build/generate-matrix.sh lts
generated 4 builds
build zone 6.1.165 for x86_64 with tags 6.1, 6.1.165 to ttl.sh/ben-ftrace/zone-k
ernel, ttl.sh/ben-ftrace/zone-kernel-sdk on ubuntu-latest
build zone 6.6.128 for x86_64 with tags 6.6, 6.6.128 to ttl.sh/ben-ftrace/zone-k
ernel, ttl.sh/ben-ftrace/zone-kernel-sdk on ubuntu-latest
build zone 6.12.75 for x86_64 with tags 6.12, 6.12.75 to ttl.sh/ben-ftrace/zone-
kernel, ttl.sh/ben-ftrace/zone-kernel-sdk on ubuntu-latest
build zone 6.18.16 for x86_64 with tags 6, 6.18, 6.18.16 to ttl.sh/ben-ftrace/zo
ne-kernel, ttl.sh/ben-ftrace/zone-kernel-sdk on ubuntu-latest

[bleggett]

Dropping our 9p doublefree patch from > 6.18.15 in favor of an equivalent upstream fix pushed in 6.18.16: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/net/9p/trans_xen.c?h=v6.18.16&id=59e7707492576bdbfa8c1dbe7d90791df31e4773

checked with @kaniini to confirm.