ap-common: Switch to Linux 6.1 by default on Arm

ap-common: Switch to Linux 6.1 by default on Arm

The "linux-arm-xen" tree currently point to Linux 5.4.17 + 3 commits:
  69117aaf07f7 ("xen/privcmd: replace kcalloc() by kvcalloc() when allocating empty pages")
  bca5bc0229a6 ("xen/privcmd: fix error handling in mmap-resource processing")
  f0f0e602f7c9 ("xen/privcmd: drop "pages" parameter from xen_remap_pfn()")

This stable version is old and doesn't build on Debian Bookworm.

The 3 extra commits are present in at least Linux 5.15, so there's
currently no reason to have a separate tree when we could use a stable
tree that osstest can update regulary.

So switch to 6.1. This will change the default Linux tree for all Arm
jobs that aren't on "linux-*" osstest branches.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>