Use Broadcast.flatten on master

[charleskawczynski]

Inspired by JuliaArrays/StaticArrays.jl#1186, this PR adds the use of Julia master's Broadcast.flatten, which has better inference properties than julia 1.10.

This may negate the need to manually apply the flattening tricks we've needed to in ClimaAtmos (cc @trontrytel), or it may just result in better inference, which seems fine to me, too.

I'm keen to see if any of our existing broken tests due to complexity heuristics start passing.

[Sbozzolo]

I don't understand this code, but I think we can trust StaticArrays. Let's see when all the tests pass.

It'd be good to add comments to explain the high-level purpose and intent of this function and when it should be used.

[charleskawczynski]

I don't understand this code, but I think we can trust StaticArrays. Let's see when all the tests pass.

Yeah, it's a bit complicated, this was just copy-pasted from julia master's Broadcast module so that StaticArrays could benefit from the bleeding edge of julia development in earlier versions of StaticArrays.

It'd be good to add comments to explain the high-level purpose and intent of this function and when it should be used.

Sure, I can add a comment with the julia/StaticArrays PR, and the issues that it fixes for us (one of which we reported)