Update Scala version in `.sdkmanrc`

[achinaou]

It would be nice if Scala Steward could update the Scala version in the .sdkmanrc as the SDKMAN became one of the official ways to install Scala.

[mzuehlke]

When adding .sdkmarc to updates.fileExtensions in the .scala-steward.conf (See https://github.com/scala-steward-org/scala-steward/blob/main/docs/repo-specific-configuration.md) the Scala version should be updated.

[achinaou]

Yes, it did!

Would it be a good idea to include it in the default list?

[mzuehlke]

Sounds like a good idea 👍 . Feel free to provided a PR.