fix deprecation warning

[tjhei]

fix:

../source/simulator/assemblers/stokes.cc:219:54: warning: ‘Number
dealii::double_contract(const dealii::Tensor<2, dim, Number>&, const
dealii::Tensor<2, dim, Number>&) [with int dim = 2; Number = double]’ is
deprecated [-Wdeprecated-declarations]
                 data.local_rhs(i) +=
(double_contract(elastic_outputs->elastic_force[q],Tensor<2,dim>(scratch.grads_phi_u[i])))

[tjhei]

I have to admit that I did not check if this is the correct contraction. @bangerth ? :-)

[tjhei]

or maybe use scalar_product?

[bangerth]

Yes, let's use scalar_product. I think that's clearer.

Pre-approved with that change!