minor: CI: don't test latest server on maint branches
minor: CI: don't test latest server on maint branches

mongodb/mongo-perl-driver,mongodb/mongo-perl-driver,dagolden/mongo-perl-driver,mongodb/mongo-perl-driver,xdg/mongo-perl-driver,dagolden/mongo-perl-driver,dagolden/mongo-perl-driver,xdg/mongo-perl-driver,dagolden/mongo-perl-driver,xdg/mongo-perl-driver