Prepare build.gradle files for Gradle 9.0

[barchetta]

Gradle 9.0 is coming in 2025:

https://gradle.org/whats-new/gradle-9/

We have a couple deprecation warnings in our build.gradle files concerning 9.0:

Build file '/Users/jdipol/GitHub/barchetta/helidon-examples/examples/quickstarts/helidon-quickstart-mp/build.gradle': line 28
The org.gradle.api.plugins.JavaPluginConvention type has been deprecated. This is scheduled to be removed in Gradle 9.0. Consult the upgrading guide for further information: https://docs.gradle.org/8.13/userguide/upgrading_version_8.html#java_convention_deprecation
        at build_kjy6ar4ydafm5o5jy0vlhovm.run(/Users/jdipol/GitHub/barchetta/helidon-examples/examples/quickstarts/helidon-quickstart-mp/build.gradle:28)
        (Run with --stacktrace to get the full stack trace of this deprecation warning.)
Build file '/Users/jdipol/GitHub/barchetta/helidon-examples/examples/quickstarts/helidon-quickstart-mp/build.gradle': line 29
The org.gradle.api.plugins.JavaPluginConvention type has been deprecated. This is scheduled to be removed in Gradle 9.0. Consult the upgrading guide for further information: https://docs.gradle.org/8.13/userguide/upgrading_version_8.html#java_convention_deprecation
        at build_kjy6ar4ydafm5o5jy0vlhovm.run(/Users/jdipol/GitHub/barchetta/helidon-examples/examples/quickstarts/helidon-quickstart-mp/build.gradle:29)
        (Run with --stacktrace to get the full stack trace of this deprecation warning.)