chore: replace deprecated Gradle configuration APIs

[devin-ai-integration]

Summary

Replace deprecated Gradle convention properties with the current java and application extension APIs while preserving Java 8 compatibility and the Hello entry point.

The LaunchDarkly Java server SDK constraint remains 7+: Maven Central and Gradle both resolve it to 7.14.0, the latest published release, so no SDK version change is needed. The application uses no deprecated LaunchDarkly SDK APIs.

Tested with:

• ./gradlew clean build --warning-mode all
• CI=true LAUNCHDARKLY_SDK_KEY=... ./gradlew run --warning-mode all (SDK initialized and evaluated sample-feature)

Link to Devin session: https://app.devin.ai/sessions/8a5b6c5b831f43098bf7440772e83035

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Note

Low Risk
Build-only Gradle API updates with no application, dependency, or SDK logic changes.

Overview
Updates Gradle build configuration to silence deprecation warnings without changing runtime behavior.

Java toolchain — Replaces the removed allprojects { sourceCompatibility / targetCompatibility } block with the java { } extension and JavaVersion.VERSION_1_8 for both source and target, preserving Java 8.

Application plugin — Renames mainClassName to mainClass on the application block so the Hello entry point stays the same.

Dependencies (LaunchDarkly server SDK 7+) and repositories are untouched.

^{Reviewed by Cursor Bugbot for commit 608b4e4. Bugbot is set up for automated code reviews on this repo. Configure here.}

[devin-ai-integration]

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