Upgrade to OpenJDK 21, Java 17 is EOL

[jordanpadams]

Summary

Upgrade the project from Java/OpenJDK 17 to OpenJDK 21.

Motivation

• Oracle JDK 17: EOL ~September 2026 (~1 month) — no free security patches after that date. https://endoflife.date/oracle-jdk
• Red Hat OpenJDK 17: EOL ~August 2027 (~1 year)
• Oracle JDK 21: EOL ~September 2028 (~2 years) — the next free LTS window

Upgrading to OpenJDK 21 buys us ~2 years before we need to act again.

Engineering Details

• Update pom.xml (or build config) to target Java 21
• Update CI/CD workflows to use openjdk:21
• Test for Java 21 compatibility issues (sealed classes, pattern matching, virtual threads, etc.)
• Update any Docker base images referencing Java 17

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