Update changelog with recent changes (#329)
Update changelog with recent changes (#329)

SDK constraint increased to 1.21.0 to support generic functions.
Iterables: enumerate is now generic.
willyp564/quiver-dart,kevmoo/quiver-dart,willyp564/quiver-dart,cbracken/quiver-dart,cbracken/quiver-dart,google/quiver-dart,kevmoo/quiver-dart