Fix Gradle warning

[rsenden]

The following warning is shown when running ./gradlew distThirdParty --warning-mode=all:

> Task :generateLicenseReport
The runtime configuration has been deprecated for resolution. This will fail with an error in Gradle 7.0. Please resolve the runtimeClasspath configuration instead. Consult the upgrading guide for further information: https://docs.gradle.org/6.8.3/userguide/upgrading_version_5.html#dependencies_should_no_longer_be_declared_using_the_compile_and_runtime_configurations

This is caused by this issue: jk1/Gradle-License-Report#161 (comment)

Once this has been fixed in the plugin, the plugin version should be updated in our build.gradle.