[TCGC]: Unify case of header names

[archerzz]

Clear and concise description of the problem

Minor.

In some cases, the name of content type header as parameter is content-type. In other cases, it's Content-Type. See https://github.com/Azure/autorest.csharp/blob/72659d42a5067035f0c2d969ed07dd523cbda4dc/test/TestProjects/FirstTest-TypeSpec/FirstTest-TypeSpec.tsp#L286

@header contentType: "text/plain",

The header name is content-type v.s. Content-Type.

This is not a big issue, because according to HTTP spec, the header name is case-insensitive.

Checklist

• [X] Follow our Code of Conduct
• [X] Check that this issue is about the Azure libraries for typespec. For feature request in the typespec language or core libraries file it in the TypeSpec repo
• [X] Read the docs.
• [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.