Chore: fix warning.

[counter2015]

This pull request includes several changes to improve code quality, update dependencies, and enhance functionality. The most important changes include updating Scala versions and dependencies, adding new compiler options, and fixing various typos and formatting issues.

Dependency Updates:

• Updated Scala versions to 3.3.3 and 2.12.19 in build.mill.
• Added scala-collection-compat dependency in build.mill.

Code Enhancements:

• Added new compiler options (-deprecation, -feature, -language:implicitConversions, -language:higherKinds) in build.mill.
• Integrated ScalafmtModule into OsModule and added collectionCompact dependency in build.mill.

Typo and Formatting Fixes:

• Corrected typos in documentation comments in os/src/ListOps.scala
• Fixed various method signatures and added missing return types in os/src/SubProcess.scala

Migration to Modern Scala Collections:

• Replaced collection.JavaConverters with scala.jdk.CollectionConverters across multiple files (os/src/Path.scala, os/src/ProcessOps.scala, os/src/ReadWriteOps.scala, os/src/ZipOps.scala, os/test/src-jvm/ZipOpJvmTests.scala, os/watch/src/WatchServiceWatcher.scala)

[counter2015]

@sake92 is there anything else I can do to improve this PR ?

[lihaoyi]

I'm going to call this a wontfix. I don't find these improvements worth the churn they introduce to the codebase