chore(backend): run on JDK 21

[JuliusHenke]

Increment B8: JDK 17 → 21 (LTS) for build, CI and the runtime image.

• `backend/build.gradle.kts`: `sourceCompatibility` and `jvmTarget` 21; Jib base `eclipse-temurin:21-jre@sha256:cff19e62…` (multi-arch index with amd64 and arm64).
• `backend-test.yml`, `backend-docker.yml`: `java-version: '21'`.
• `backend/README.md`: JDK >= 21 for local development.

Deployment notes

• The runtime image now runs Java 21. No config changes. Watch memory and GC after the first deploy (default GC stays G1).
• Rollback: redeploy the previous image tag.

Verification

• `./gradlew test` on JDK 21.0.12: 71 tests, 0 failures.
• Built the image locally (`jibDockerBuild`) and started it against Postgres 18: Java 21.0.12, runs as UID 1000, Tomcat 10.1, app started in about 6s, Flyway ran.
• Not verified: the CI workflows (will run on this PR) and arm64 image execution.

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