Make ExecutionSpace::concurrency() a non-static member function

[dalg24]

ExecutionSpace::concurrency() is currently declared as a static member function in all backends and this is an issue because the concurrency is really a property of an execution space instance, not from its type.

This is something we knew we would have to deal with for a while but we can't procrastinate much longer.
In particular, it is problematic as we add support for execution space instances to parallel host backends.
Apropos, #5105 attempts to work around that issue by providing OpenMP::concurrency(OpenMP const& = OpenMP()) -> int

The change from static to non-static member function is a breaking change but the fix on the user-side is rather trivial

int concurrency = Kokkos::DefaultExecutionSpace::concurrency();  // does not compile any more
// change to
int concurrency = Kokkos::DefaultExecutionSpace().concurrency();  // using default-constructed execution space
// or
int concurrency = execution_space.concurrency();  // calling member function on an instance

The intent of this issue is to announce our intent to make this change and to formulate a plan for the transition.

[PhilMiller]

4.0 would be a really nice timeframe to make such a change. We didn't deprecate to provide a warning or anything, but I think users would survive without too much grumbling if we did it ASAP

[dalg24]

Unfortunately I can't think of any way to emit warnings because we would not be able to "overload" on the static member function.