apply plugin: 'org.junit.platform.gradle.plugin' を追加したら以下のようなエラーが出た。

FAILURE: Build failed with an exception.

* Where:
Build file '/Users/eiel/Documents/ghq/github.com/junit-team/junit5-samples/junit5-gradle-consumer/build.gradle' line: 23

* What went wrong:
A problem occurred evaluating root project 'junit5-gradle-consumer'.
> org/gradle/api/plugins/JavaBasePlugin

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

* Get more help at https://help.gradle.org

~/.gradle を削除したら、治った。