gitlab-ci: check-dco.py: switch from master to stable-7.2 branch

gitlab-ci: check-dco.py: switch from master to stable-7.2 branch

There's one commit, tagged v7.2.2, without Signed-off-by line.
Due to this, check-dco test always fail on 7.2.  Since this is
a stable branch with almost all commits coming from master
already with S-o-b (except of the version bumps and very rare
stable-specific commits), and v7.2.2 is already cast in stone,
let's base the check on stable-7.2 branch (with its last version)
instead of master branch.  This way, staging-7.2 will be checked
against stable-7.2, but stable-7.2 itself will not be checked
anymore, - so we can catch errors during stable preparations.

Note: this is a change specific to stable-7.2 branch/series, it
is not supposed to be in master.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Thomas Huth <thuth@redhat.com>