Specialize setindex! for VectorOfArray{StructArray}

[jlchan]

Addresses SciML/OrdinaryDiffEq.jl#2625 (comment)

The issue is that VectorOfArray indexing will not mutate a VectorOfArray{StructArray} due to the way StructArrays treats assignment: https://juliaarrays.github.io/StructArrays.jl/stable/counterintuitive/#Modifying-a-field-of-a-struct-element. For example:

u = VectorOfArray(StructArray{MVector{1, Float64}}(ntuple(_ -> [1.0, 2.0], 1)))
u[1,1] = 100
@show u[1,1] # this is still 1.0

[jlchan]

I don't think Downgrade and Format CI failures are related to this PR.