adopt `@IgnoreEmptyDirectories` when Gradle 6.8+ becomes baseline

Just a tracking issue for if/when 6.8 or higher becomes our baseline. Happy to take a PR which uses feature-detection to enable this now with whatever versions [we currently support](https://github.com/diffplug/spotless/tree/main/plugin-gradle#requirements).

https://docs.gradle.org/6.8-rc-1/javadoc/org/gradle/api/tasks/IgnoreEmptyDirectories.html

Start by reviewing the Gradle requirements linked in the issue and the Gradle 6.8 documentation for @IgnoreEmptyDirectories. Determine whether the change should wait for a new baseline or use feature detection while current versions remain supported. Done means the annotation is adopted without breaking the supported Gradle versions.

Written by the indexing model from the issue text.