[examples] Use P2 elements in adaptive example

In order to demonstrate the (hanging-node) constraints
work with higher order bases, too, use 2nd order FEM
as ansatz space and modify the problem such that the
boundary data are not representable exactly in the FE
space.

As extension space we use 4th order finite elements.
By fixing certain DOFs we still get incremental
bubbles wrt. to the P2-basis.