update Gradle snippet on SOURCE_DATE_EPOCH page

https://reproducible-builds.org/docs/source-date-epoch/#gradle

The example should be updated to use Kotlin (default for over 2 years), configuration-avoidance and include dirPermissions and filePermissions:

tasks.withType<AbstractArchiveTask>().configureEach {
    isPreserveFileTimestamps = false
    isReproducibleFileOrder = true
    dirPermissions {
        unix("rwxr-xr-x")
    }
    filePermissions {
        unix("rw-r--r--")
    }
}

Maybe also add a link to: https://docs.gradle.org/current/userguide/working_with_files.html#sec:reproducible_archives

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More information: https://github.com/gradle/gradle/issues/33296