ci: use latest Ubuntu release

ci: use latest Ubuntu release

Both GitHub Actions and GitLab CI use the "ubuntu:latest" tag as the
default image for most jobs. This tag is somewhat misleading though, as
it does not refer to the latest release of Ubuntu, but to the latest LTS
release thereof. But as we already have a couple of jobs exercising the
oldest LTS release of Ubuntu that Git still supports, it would make more
sense to test the oldest and youngest versions of Ubuntu.

Adapt these jobs to instead use the "ubuntu:rolling" tag, which refers
to the actual latest release, which currently is Ubuntu 24.10.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>