Upgrade Java version from 11 to 17

[devin-ai-integration]

Upgrade Java version from 11 to 17

Summary

This PR upgrades the project's Java version from 11 to 17 by updating build configuration and CI pipeline. The changes include:

• Updated sourceCompatibility and targetCompatibility in build.gradle from '11' to '17'
• Updated GitHub Actions workflow to use JDK 17 instead of JDK 11
• Updated GitHub Actions dependencies from v2 to v3 (checkout, setup-java, cache) to fix deprecation errors

Spring Boot 2.6.3 (currently used in this project) officially supports Java 17, so no dependency updates are required for this upgrade. CI tests have passed successfully.

Review & Testing Checklist for Human

• Run the application locally with Java 17 and verify it starts up without errors. CI tests passing only validates unit tests, not actual application runtime.
• Test key user flows (registration, login, article CRUD, commenting, following) to ensure no behavioral changes with Java 17
• Check application logs for any Java 17-related warnings or deprecated API usage that might not cause immediate failures
• Consider deploying to a staging environment for integration testing before production

Notes

• While CI tests passed, they only run unit tests (./gradlew clean test). I haven't verified that the full application starts and runs correctly with Java 17.
• The GitHub Actions version updates (v2 → v3) were necessary to fix a deprecation error that was blocking CI, but are beyond the original scope.

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Link to Devin run: https://app.devin.ai/sessions/f7ca513cd111465a95a94c9b6180d175
Requested by: Shawn Azman (shawn@cognition.ai) / @ShawnAzman

[devin-ai-integration]

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