Migrate from Java 11 to Java 17

[devin-ai-integration]

Summary

Targets Java 17: sourceCompatibility/targetCompatibility '11' → '17' in build.gradle, README requirement updated.

Compatibility audit (no bumps needed): Gradle wrapper 7.4 (17 supported since 7.3), Spring Boot 2.6.3, Lombok 1.18.22 (Boot-managed), DGS codegen 5.0.6.

One Java 17 issue surfaced: Spotless 6.2.1's google-java-format fails on JDK 17 with IllegalAccessError ... module jdk.compiler does not export com.sun.tools.javac.parser (also with googleJavaFormat('1.17.0')). Fixed by adding gradle.properties with org.gradle.jvmargs=--add-exports=jdk.compiler/com.sun.tools.javac.{api,file,parser,tree,util}=ALL-UNNAMED. DefaultJwtServiceTest was reformatted to satisfy spotlessJavaCheck.

Verified: JAVA_HOME=<jdk17> ./gradlew clean build → BUILD SUCCESSFUL, 68 tests, 0 failures.

Note: .github/workflows/gradle.yml no longer exists on master (removed in #411), so there is no CI JDK step to update.

Devin-Org: engineering

Link to Devin session: https://app.devin.ai/sessions/d90064582e4b4f90a726cea0989b432a
Open in Devin Desktop: https://app.devin.ai/desktop/session/d90064582e4b4f90a726cea0989b432a?variant=devin
Requested by: @sophialyssy-demo

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Devin errored when opening this Pull Request as sophialyssy-demo.
As a fallback, Devin opened this PR as itself.

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.