Upgrade project from Java 11 to Java 17

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Summary

Moves the build to Java 17 (sourceCompatibility/targetCompatibility 11 -> 17).

The only real incompatibility was Spotless: googleJavaFormat() at Spotless 6.2.1 resolves an old google-java-format that reaches into com.sun.tools.javac.*, which JDK 16+ no longer opens, so spotlessJavaCheck failed with an access error before any compilation problem. Fixed by bumping the plugin to 6.25.0 and pinning googleJavaFormat('1.15.0'); the newer formatter rewraps one line in DefaultJwtServiceTest (the entire formatting churn from the bump).

Everything else was already Java 17-clean at its current version — no changes needed to Spring Boot 2.6.3, DGS codegen 5.0.6 / graphql-dgs-spring-boot-starter:4.9.21, Lombok (1.18.22 via the Boot BOM), sqlite-jdbc:3.36.0.3, or the Gradle 7.4 wrapper.

Verified on OpenJDK 17.0.19 / Gradle 7.4: generateJava succeeds, clean build passes with 68 tests green, and bootRun starts (Started RealWorldApplication, GET /tags -> 200).

Devin-Org: engineering

Link to Devin session: https://app.devin.ai/sessions/201fafd3ea2a418cb34e89b0e9a7a1c5
Open in Devin Desktop: https://app.devin.ai/desktop/session/201fafd3ea2a418cb34e89b0e9a7a1c5?variant=devin

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