No fully-compatible MPI implementations on Ubuntu 22.04 LTS

[roystgnr]

OpenMPI has a fix for their regression currently posted at https://github.com/open-mpi/ompi/pull/10527, and MPICH seems to already have their git head working again, but in the meantime, not being able to do an MPI_MIN with an arbitrarily large unsigned long is a big deal for us. At the very least it makes DistributedMesh unusable with METHOD=dbg (with our git head I get failures from the semiverify asserted at src/mesh/mesh_tools.C, line 2164), and there's no telling where else we might be triggering the same problem.

I'm currently working around the problem myself with a --download-mpich PETSc build (which pulls MPICH 3.4.1 right now; that seems to predate the regression there), just posting a warning for others.