Fixes a bit shift issue in an MPI constant

[derobins]

Redefines TWO_GIG_LIMIT to be INT32_MAX instead of 1 << 31, which
fixes an issue revealed by compiling with -Wshift-overflow.

[derobins]

Fixes this warning:

../../develop/src/H5mpi.c: In function ‘H5_mpi_set_bigio_count’:
../../develop/src/H5mpi.c:28:26: warning: result of ‘1 << 31’ requires 33 bits to represent, but ‘int’ only has 32 bits [-Wshift-overflow=]
28 | #define TWO_GIG_LIMIT (1 << 31)
| ^~
../../develop/src/H5mpi.c:56:50: note: in expansion of macro ‘TWO_GIG_LIMIT’
56 | if ((new_count > 0) && (new_count < (hsize_t)TWO_GIG_LIMIT)) {