[example][python][bugfix] Fix quadrature order in poisson-pq2.py

When using Q4 ansatz functions, the product of gradients is still a 4th order polynomial. Hence we should use order 4 for integration. While the solution looks ok with order 3, it is actually slightly different and one would probably not get the correct 2nd order convergence.

added bugfix label

enabled an automatic merge when the pipeline for cd766c17 succeeds

mentioned in commit f669be26

merged