MFEM deprecation warnings

Not sure how many there were but there are deprecation warnings in the Smith build coming from here.

Example:

```
smith/repo/ContinuationSolvers/continuationsolvers/problems/ObstacleProblems.cpp:54:10: warning: 'EliminateVDofsInRHS' is deprecated [-Wdeprecated-declarations]
   54 |   Kform->EliminateVDofsInRHS(ess_tdof_list, xDC, f);
      |          ^
```