Remove messaging header normalization

[trask]

Currently messaging headers attribute keys are normalized, replacing - with _.

This normalization was removed in HTTP semconv prior to its stabilization, and we should do the same in message headers.

Since this will be a breaking change, we should defer it to the 3.0 release.

[AlchemyDing]

@trask I think it's about modifying this line and related unit tests, right?