Implicit dependency on latest Gradle

[henryptung]

What happened?

1. Excavator updates Gradle wrapper version to latest.
2. Plugin builds using gradleApi() dependency (both directly declared and implicitly declared via java-gradle-plugin)
3. Plugin uses APIs that only exist in later Gradle (e.g. Provider.orElse(), since Gradle 5.6)
4. Plugin fails in older Gradle 5.x projects (e.g. 5.2)

What did you want to happen?

Plugin should conform to version requirements as laid out in release notes (>= 5.1) and should not implicitly bump with every release.

Suggestions

• Don't use Excavator to bump Gradle wrapper version
• Replace gradleApi() dependency (and java-gradle-plugin) with an explicit dependency on a fixed version of Gradle API

[iamdanfox]

(One more option is to run tests against the specific gradle version we intended to maintain compatibility with)