Error out if `ignoreUnsupportedConstraintTraits` has no effect (#2539)

Now that constraint traits are supported in server SDKs (with some
corner case caveats, see #1401), this flag will almost always be useless
for those early adopters of constraint traits. It is thus convenient to
inform the user that they should remove it.

See
https://github.com/awslabs/smithy-rs/pull/2516#issuecomment-1490393056.

## Checklist
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- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates

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