[tcgc] switch from `experimental_sdkPackage` to `sdkPackage`

[iscai-msft]

Clear and concise description of the problem

We've had the prefix experimental_ for a while, signifying that we will be changing this name. I think it's time in this release to just change it to sdkPackage

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.