qa/cephadm: fix test_repos.sh for jammy nodes

qa/cephadm: fix test_repos.sh for jammy nodes

The test uses add-repo --release 17.2.6 to verify version-string repo
handling, but debian-17.2.6 only has focal and bullseye suites and
jammy packages weren't built until 17.2.7. This causes apt-get update
to fail with a 404 on ubuntu_22.04 nodes.

see: https://download.ceph.com/debian-17.2.6/dists/

This fix bumps the version to 17.2.7 which includes a jammy suite.

Fixes: https://tracker.ceph.com/issues/77130
Signed-off-by: Redouane Kachach <rkachach@ibm.com>