Java 21 migration — Gradle build config

[devin-ai-integration]

Summary

Updates the Gradle build configuration to support the Java 8 → Java 21 / Spring Boot 2.0.2 → 3.2.5 migration. This PR is scoped to Gradle build config only — pom.xml, the Maven wrapper, and .java sources are owned by sibling PRs.

Changes:

• build.gradle
  • Bump org.springframework.boot:spring-boot-gradle-plugin from 2.0.2.RELEASE to 3.2.5.
  • Bump sourceCompatibility and targetCompatibility from 1.8 to 21.
  • Replace deprecated compile(...) with implementation(...) and testCompile(...) with testImplementation(...).
  • Rename bootJar { baseName / version } to bootJar { archiveBaseName / archiveVersion } (required by modern Gradle).
• gradle/wrapper/gradle-wrapper.properties
  • Bump distributionUrl from gradle-4.6-bin.zip to gradle-8.7-bin.zip (Gradle 8.5+ is required for Java 21).

Review & Testing Checklist for Human

• Verify the integration PR (combining this PR with the Maven-config and Java-source migration PRs) builds successfully against Java 21 with ./gradlew build.
• Confirm ./gradlew bootJar still produces an artifact named gs-spring-boot-0.1.0.jar after the archiveBaseName / archiveVersion rename.
• Sanity-check that no other Gradle build files exist in the repo that also reference compile/testCompile or the old plugin version.

Notes

• This PR alone will not produce a working build — it must be merged together with the sibling PRs (Maven build config, Java source updates) on top of devin/1777913688-java21-migration-base.
• Per the task spec, no lint/test was run in this child session; CI on the integration PR is the source of truth.

Link to Devin session: https://app.devin.ai/sessions/af6995a13ace421a845a182fbdc7759e
Requested by: @dr-phil

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✅ Devin Review: No Issues Found

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

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