⬆️ Upgrade gradle to 8.4

[nvervelle]

Upgrade gradle to 8.4 and enable detailed warnings about deprecated gradle usage.

I will try to do another PR to fix the warnings about the deprecation of JavaPluginConvention.

Build file '/home/nvervelle/dev/Tests/opa-gradle-plugin/build.gradle': line 12
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/8.4/userguide/upgrading_version_8.html#java_convention_deprecation
        at build_28vphb26scj7alubxe8loxln3.run(/home/nvervelle/dev/Tests/opa-gradle-plugin/build.gradle:12)
        (Run with --stacktrace to get the full stack trace of this deprecation warning.)
Build file '/home/nvervelle/dev/Tests/opa-gradle-plugin/build.gradle': line 12
The org.gradle.api.plugins.Convention 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/8.4/userguide/upgrading_version_8.html#deprecated_access_to_conventions
        at build_28vphb26scj7alubxe8loxln3.run(/home/nvervelle/dev/Tests/opa-gradle-plugin/build.gradle:12)
        (Run with --stacktrace to get the full stack trace of this deprecation warning.)
Build file '/home/nvervelle/dev/Tests/opa-gradle-plugin/build.gradle': line 13
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/8.4/userguide/upgrading_version_8.html#java_convention_deprecation
        at build_28vphb26scj7alubxe8loxln3.run(/home/nvervelle/dev/Tests/opa-gradle-plugin/build.gradle:13)
        (Run with --stacktrace to get the full stack trace of this deprecation warning.)
Build file '/home/nvervelle/dev/Tests/opa-gradle-plugin/build.gradle': line 13
The org.gradle.api.plugins.Convention 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/8.4/userguide/upgrading_version_8.html#deprecated_access_to_conventions
        at build_28vphb26scj7alubxe8loxln3.run(/home/nvervelle/dev/Tests/opa-gradle-plugin/build.gradle:13)
        (Run with --stacktrace to get the full stack trace of this deprecation warning.)