Deprecate BoxDecoration.shape #108051

Discussion here: #103833

According to @Hixie:

[BoxShape.oval] There is a comment saying new shapes shouldn't go here.

I should probably have included more information in the comment saying why:
Each new enum value is a lot of new code in a bunch of places, including in code we don't control (e.g. app code, that's why the Google tests are failing), so we'd need to write a migration guide and migrate a bunch of code.

• Each new enum value requires new code in a bunch of places.
• That new code likely won't get tree-shaken out, since I don't think the compiler can know which values are never used. So this has a permanent binary size cost even for people who don't use it.
• In principle, we've moved to ShapeBorders rather than BoxShape for new shapes, and the existence of BoxShape is just a legacy feature at this point. Adding more shapes encourages people to use BoxShape in the future.

So I think it is worth trying to remove it. We can't deprecate BoxShape completely because InkWell uses it. But we can remove from BoxDecoration.