Migrate to Java 21 and Scala 2.13.18

[dwoot-verily]

• Update Java from 8/11 to 21
• Update Scala to 2.13.18 with 2.12.20 for sbt plugin compatibility
• Update sbt to 1.10.8
• Replace deprecated GC flags with G1GC
• Update compiler targets to Java 21
• Add Java module reflection access flags for tests
• Update sbt plugin to use Scala 2.12.20

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}