[mkl,eigen] Refactored interdependent setup.

- [cmake] Also use MKLs OpenMP implementation for serial configs.
- [cmake] Renamed OGS_USE_PETSC_MKL_EIGEN_OPENMP to OGS_USE_MKL_EIGEN_OPENMP.
- [cmake] Option OGS_EIGEN_PARALLEL_BACKEND replaces other configs.
  Removed:
  - OGS_USE_MKL_EIGEN_OPENMP
  - OGS_EIGEN_USE_MKL
- Also when OGS_USE_MKL=ON then OGS_EIGEN_PARALLEL_BACKEND defaults to
MKL.
- Updated eigen-unsupported version to inlude fix for MKL backend.
  Fix: https://gitlab.com/libeigen/eigen/-/merge_requests/851
- Moved OGS_EIGEN_PARALLEL_BACKEND initialization.
- [cmake] Only link to OpenMP when eigen backend is set to OpenMP.
- [cmake] Eigen includes OpenMP or MKL depending on backend.