Upgrade Java version from 11 to 17

[devin-ai-integration]

Summary

Updates the project's Java version from 11 to 17 across build configuration, CI pipeline, and documentation. No dependency versions were changed — Spring Boot remains at 2.6.3.

Files changed:

• build.gradle — sourceCompatibility and targetCompatibility updated from '11' to '17'
• .github/workflows/gradle.yml — JDK setup step changed from java-version: '11' to '17'
• README.md — "You'll need Java 11 installed" → "You'll need Java 17 installed"

All 68 tests passed locally against Java 17.

Review & Testing Checklist for Human

• Verify CI passes with JDK 17 (local test pass is necessary but not sufficient)
• Smoke test ./gradlew bootRun with Java 17 and hit http://localhost:8080/tags — Java 17's stricter internal API encapsulation can surface reflection issues at runtime that unit tests don't catch (relevant for Spring Boot 2.6.3 + MyBatis + DGS)

Notes

• Spring Boot 2.6.3 officially supports Java 17, but no Spring Boot version bump was made. If any --add-opens or illegal-access warnings appear at runtime, a Spring Boot upgrade may be needed as a follow-up.
• Requested by: @sanch-cognition
• Link to Devin run

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[devin-ai-integration]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

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

View in Devin Review to see 3 additional findings.