update .scalafmt.conf. enforce new wildcard syntax

[xuwei-k]

Pull Request Checklist

• Have you read through the contributor guidelines?
• Have you squashed your commits?
• Have you added copyright headers to new files?
• Have you updated the documentation?
• Have you added tests for any changed functionality?

Fixes

Purpose

Background Context

old wildcard syntax [_] is deprecated in latest Scala 3.

Welcome to Scala 3.6.3 (21.0.5, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                                                                                           
scala> val a: List[_] = Nil
1 warning found
-- Warning: --------------------------------------------------------------------
1 |val a: List[_] = Nil
  |            ^
  |        `_` is deprecated for wildcard arguments of types: use `?` instead
val a: List[?] = List()
                                                                                                                                           
scala> val b: List[?] = Nil
val b: List[?] = List()

References