[ci] Update the ci to use dune 2.9
What this MR does / why does DuMux need it:
It updates the CI gitlab to test dumux master with dune 2.9 instead of 2.8 / DuMux 3.7 will not support dune 2.8
Fixes #1226 (closed)
Edited by Timo Koch
What this MR does / why does DuMux need it:
It updates the CI gitlab to test dumux master with dune 2.9 instead of 2.8 / DuMux 3.7 will not support dune 2.8
Fixes #1226 (closed)