Instructions for disabling OpenMPI if version >= 3

[viktor76525]

OpenMPI version 4 is available and 3 seems to be required for some things such as cuda.
Therefore some people may be using a newer version. Errors occur with openmpi >= 3.0. Won't build even if it is optional.

ue_mpi.c: In function ‘mpi_register_octant’:                                                  
ue_mpi.c:66:3: error: static assertion failed: "MPI_Type_struct was removed in MPI-3.0.  Use MPI_Type_create_struct instead."                                                                
   66 |   MPI_Type_struct(num_fields, blocklen, disp, ftype, dt);
      |   ^~~~~~~~~~~~~~~                                                                                                                                                                    
ue_mpi.c: In function ‘mpi_register_dispatch’:                                                                                                                                               
ue_mpi.c:93:3: error: static assertion failed: "MPI_Type_struct was removed in MPI-3.0.  Use MPI_Type_create_struct instead."                                                                
   93 |   MPI_Type_struct(num_fields, blocklen, disp, ftype, dt);              
      |   ^~~~~~~~~~~~~~~