feat: cross-compile for Scala 2.12 + 2.13 with Java 17 target

[fupelaqu]

• Add crossScalaVersions (2.12.20, 2.13.16) with conditional scalacOptions
• Replace akron with cron-utils (pure Java, cross-compile friendly)
• Migrate Configs → ConfigReader for Scala 2.13 compatibility
• Bump generic-persistence-api 0.7.3 → 0.8.1, scalapb-extensions 0.1.7 → 0.2.0
• Upgrade sbt 1.7.1 → 1.8.3, Docker base image to eclipse-temurin:17-jdk
• Update GitHub Actions to v4, JDK 17, add cross-compile CI step
• Fix Scala 2.13 postfix notation and exhaustive match warnings
• Version bump 0.7.3 → 0.8-SNAPSHOT

Closed Issue #7

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 64.51%. Comparing base (9d6f0db) to head (9d4fefb).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
...pp/softnetwork/scheduler/api/SchedulerServer.scala	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #8      +/-   ##
==========================================
- Coverage   65.61%   64.51%   -1.10%     
==========================================
  Files          17       31      +14     
  Lines         474      589     +115     
  Branches        0      126     +126     
==========================================
+ Hits          311      380      +69     
- Misses        163      209      +46     

Flag	Coverage Δ
unittests	64.51% <80.00%> (-1.10%)	⬇️

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