chore: standardize JDK 17 toolchain

[nawforce]

Summary

• align the Maven publishing workflow with the build workflow on JDK 17
• replace JVM javac -source 8 -target 8 with javac --release 8
• add Scala 2.13 -release 8 to the JVM project only, leaving Scala.js compiler settings unchanged

This makes build and publishing consistent while enforcing Java 8 platform API and bytecode compatibility for JVM artifacts.

No changelog entry is included because this changes build/CI enforcement and not user-facing behavior.

Validation

Using JDK 17.0.20:

• sbt build
• SAMPLES=/Users/kevinjones/adt/apex-samples sbt test after restarting the pre-existing sbt server so it inherited the environment; 6,314 JVM tests and 52 Scala.js tests passed
• inspected all 92 emitted production JVM class files: every class has major version 52

The pinned sample checkout remained clean and detached at v1.4.0.