Cross-compile generic-scheduler-api for Scala 2.12 and 2.13

[fupelaqu]

Summary

Cross-compile generic-scheduler-api to support both Scala 2.12 and 2.13, upgrading from JDK 8 to JDK 17.

Changes

• Build: Cross-compile Scala 2.12.20 + 2.13.16, sbt 1.7.1 → 1.8.3, JDK 8 → 17
• Dependencies: Replace akron (Scala-specific) with cron-utils (Java), upgrade scalapb-extensions 0.1.7 → 0.2.0, generic-persistence 0.7.3 → 0.8.1, add scala-collection-compat
• Config: Migrate Configs → ConfigReader (Scala 2.13 compatible)
• CI/CD: GitHub Actions v3 → v4, ubuntu-latest runners, JDK 17, cross-compile step
• Docker: Base image openjdk:8 → eclipse-temurin:17-jdk
• Code: Fix Scala 2.13 postfix operator warnings, add exhaustive match in SchedulerServer

Context

Part of Story 5.10-pre-2-a in the SoftClient4ES licensing infrastructure epic. Required as a prerequisite for the scheduler integration in the licensing refresh module.