qa/distros: ask for Rocky by major, not a point release

Rocky only maintains its newest point release and the rocky packages
shaman builds are compiled on it -- ceph-osd-crimson now needs
c-ares >= 1.28, which only 10.2 ships -- so the lab treats Rocky like
CentOS Stream: FOG images track the latest minor and jobs ask for
os_version "10" (and "9").  Pinning the suite to "10.1" either fails the
deploy (no 10.1 image) or, as on 2026-08-19, fails every package install
with "nothing provides c-ares(x86-64) >= 1.28.0".

Needs teuthology PR #2246 (major aliases in the codename map, and FOG
resolves a major-only version to the latest available image and verifies
the deployed OS against it).

Fixes: https://tracker.ceph.com/issues/79682

Signed-off-by: David Galloway <david.galloway@ibm.com>