Migration guide 2.3: Missing Json error keys renaming

Hello,

I've just found that many JSON error keys have been renamed between 2.2 and 2.3. These break changes are not mentioned in the Migration Guide.

There are two commits involved:

- https://github.com/playframework/playframework/commit/5b0f70860d329d26aadd47f77547682589c30052
- https://github.com/playframework/playframework/commit/ebe6e017f661355358fcbda939b660f7bcac4ef4