Update thermal solid input file

Initially, it complains about having different polynomial interpolation orders between solid and heat transfer modules, but when I set them both to one, I get:

```
0: MFEM abort: SuperLUSolver: Found a singular matrix, U(210,210) is exactly zero!
0: ... in function: void mfem::SuperLUSolver::HandleError(int) const
0: ... in file: linalg/superlu.cpp:754
```

https://github.com/LLNL/serac/blob/cc3e357c7d2e39fb8a2b522a1624d0378a9ade2e/data/input_files/tests/thermal_solid/thermal_expansion.lua#L36

Start with data/input_files/tests/thermal_solid/thermal_expansion.lua at line 36 and reproduce the reported polynomial-order warning and SuperLU singular-matrix failure. Trace the thermal solid setup and determine what input-file change removes the failure; done means the updated input no longer triggers the reported error.

Written by the indexing model from the issue text.