Update vec-initializer-lists.md changelog to be vec-constexpr.md

The entire changelog for vec-initializer-lists.md is now out of date as we removed intializer lists and replaced it with constexpr and variadic constructors.

We need to go over all the examples provided in this changelog and make sure that they still work with vtk-m. Plus we need to show how to make constexpr vtkm::Vec<> types.