message: Add CHANGELOG for ContractResult/SystemResult serialization
diff: **all**
  - Drop support for Rust versions lower than 1.45.2.
  +- The serialization of the result from `init`/`migrate`/`handle`/`query` changed
  + in an incompatible way. See the new `ContractResult` and `SystemResult` types
  + and their documentation.
  **cosmwasm-std**
  