Update mentions to codegen configuration key in changelog (#2166)

    Upon receiving a request that violates the modeled constraints, the server SDK will reject it with a message indicating why.

    Unsupported (constraint trait, target shape) combinations will now fail at code generation time, whereas previously they were just ignored. This is a breaking change to raise awareness in service owners of their server SDKs behaving differently than what was modeled. To continue generating a server SDK with unsupported constraint traits, set `codegenConfig.ignoreUnsupportedConstraints` to `true` in your `smithy-build.json`.

    Unsupported (constraint trait, target shape) combinations will now fail at code generation time, whereas previously they were just ignored. This is a breaking change to raise awareness in service owners of their server SDKs behaving differently than what was modeled. To continue generating a server SDK with unsupported constraint traits, set `codegen.ignoreUnsupportedConstraints` to `true` in your `smithy-build.json`.

    ```json

    {

        ...

        "rust-server-codegen": {

            ...

            "codegen": {

                "ignoreUnsupportedConstraints": true

            }

        }

    }

    ```

- ⚠🎉 (server, [smithy-rs#1342](https://github.com/awslabs/smithy-rs/issues/1342), [smithy-rs#1119](https://github.com/awslabs/smithy-rs/issues/1119)) Server SDKs now generate "constrained types" for constrained shapes. Constrained types are [newtypes](https://rust-unofficial.github.io/patterns/patterns/behavioural/newtype.html) that encapsulate the modeled constraints. They constitute a [widespread pattern to guarantee domain invariants](https://www.lpalmieri.com/posts/2020-12-11-zero-to-production-6-domain-modelling/) and promote correctness in your business logic. So, for example, the model:

    ```smithy

    Constrained types _guarantee_, by virtue of the type system, that your service's operation outputs adhere to the modeled constraints. To learn more about the motivation for constrained types and how they work, see [the RFC](https://github.com/awslabs/smithy-rs/pull/1199).

    If you'd like to opt-out of generating constrained types, you can set `codegenConfig.publicConstrainedTypes` to `false`. Note that if you do, the generated server SDK will still honor your operation input's modeled constraints upon receiving a request, but will not help you in writing business logic code that adheres to the constraints, and _will not prevent you from returning responses containing operation outputs that violate said constraints_.

    If you'd like to opt-out of generating constrained types, you can set `codegen.publicConstrainedTypes` to `false`. Note that if you do, the generated server SDK will still honor your operation input's modeled constraints upon receiving a request, but will not help you in writing business logic code that adheres to the constraints, and _will not prevent you from returning responses containing operation outputs that violate said constraints_.

    ```json

    {

        ...

        "rust-server-codegen": {

            ...

            "codegen": {

                "publicConstrainedTypes": false

            }

        }

    }

    ```

- 🐛⚠🎉 (server, [smithy-rs#1714](https://github.com/awslabs/smithy-rs/issues/1714), [smithy-rs#1342](https://github.com/awslabs/smithy-rs/issues/1342)) Structure builders in server SDKs have undergone significant changes.

    The API surface has been reduced. It is now simpler and closely follows what you would get when using the [`derive_builder`](https://docs.rs/derive_builder/latest/derive_builder/) crate:

            if (canBeIgnored) {

                msg += """

                    If you want to go ahead and generate the server SDK ignoring unsupported constraint traits, set the key `ignoreUnsupportedConstraints`

                    inside the `runtimeConfig.codegenConfig` JSON object in your `smithy-build.json` to `true`."""

                    inside the `runtimeConfig.codegen` JSON object in your `smithy-build.json` to `true`."""

            }

            return msg.trimIndent().replace("\n", " ")

        }