update `.scalafmt.conf`. enforce new wildcard syntax

[xuwei-k]

Pull Request Checklist

• [x] Have you read How to write the perfect pull request?
• [x] Have you read through the contributor guidelines?
• [x] Have you referenced any issues you're fixing using commit message keywords?
• [x] Have you added copyright headers to new files?
• [x] Have you checked that both Scala and Java APIs are updated?
• [x] Have you updated the documentation for both Scala and Java sections?
• [x] Have you added tests for any changed functionality?

Helpful things

Fixes

Purpose

Background Context

• Scala 3 compiler report warnings since 3.4 https://github.com/lampepfl/dotty/pull/18813
• Scala 2.12 and 2.13 support new syntax without -Xsource:3 option https://github.com/scala/scala/pull/10005

References

• https://scalameta.org/scalafmt/docs/configuration.html#rewritescala3converttonewsyntax