Make MPItrampoline the default

I'll admit to initially being a bit reluctant to making MPItrampline the default MPI implementation, but given
a. it has so far proven fairly successful, and
b. the situation with binary dependencies _without_ MPItrampoline is somewhat difficult (https://github.com/JuliaPackaging/Yggdrasil/issues/6893).
I think it might be worth now making the switch. Any thoughts?

I think my main questions are:
- are there any platforms/MPI libraries it doesn't work with?
- what would happen "out of the box"? I like that people don't have to install their own MPI implementation for single node stuff.
- could we make it easier to build MPIwrapper? e.g. install it in a https://github.com/JuliaPackaging/Scratch.jl space?

cc @eschnett @lcw