Render only shape name in server error responses (#2866)

This essentially reverts the behavior introduced in #1982.

The rationale for that change:

> [...] since some existing clients rely on it to deserialize the error
shape
> and fail if only the shape name is present

no longer holds. Since serializing only the shape name is a SHOULD in
the spec, it's best that we honor it.

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._