Upgrade Scala to 3.7.1

Update Scala version from 2.12.11 to 3.7.1. Scala is only used for testing dependencies (airframe-log) and build tooling, not for main application code. All tests pass with the new version.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: xerial

build.sbt

@@ -27,7 +27,7 @@ ThisBuild / dynverSonatypeSnapshots := true
 // Use coursier friendly version separator
 ThisBuild / dynverSeparator := "-"
 
-ThisBuild / scalaVersion := "2.12.11"
+ThisBuild / scalaVersion := "3.7.1"
 
 // For building jars for JDK8
 ThisBuild / javacOptions ++= {