Fix missing setProperty error when using Gradle 6

[fawind]

Before this PR

With #1875, baseline is build using Gradle 7 and transitively Groovy 3.x.
As a result, projects that still use Gradle 6 and thus Groovy 2.x fail with the following error when applying the BaselinEclipse or BaselineIdea plugin (both are the only plugins that are still Groovy and not Java classes) (example):

Caused by: java.lang.AbstractMethodError: Receiver class com.palantir.baseline.plugins.BaselineEclipse does not define or inherit an implementation of the resolved method 'abstract void setProperty(java.lang.String, java.lang.Object)' of interface groovy.lang.GroovyObject.
	at com.palantir.baseline.plugins.BaselineEclipse.apply(BaselineEclipse.groovy:69)
	at com.palantir.baseline.plugins.BaselineEclipse.apply(BaselineEclipse.groovy)
	at org.gradle.api.internal.plugins.ImperativeOnlyPluginTarget.applyImperative(ImperativeOnlyPluginTarget.java:43)

Haven't gotten completely on the bottom of this but the issue seems to be that going from Groovy 2.x to 3.x GroovyObjectSupport is no longer provided per default.

Relevant issue: https://issues.apache.org/jira/browse/GROOVY-4855

After this PR

Adding extends GroovyObjectSupport adds default implementations of Groovy methods including setProperty.

Tested it on two internal repos and compiled again.

==COMMIT_MSG==
Fix missing setProperty error when using Gradle 6
==COMMIT_MSG==

[changelog-app]

Generate changelog in changelog/@unreleased

Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

Fix missing setProperty error when using Gradle 6

Check the box to generate changelog(s)

• Generate changelog entry

[fawind]

Want to sanity double check this on CI quickly 👍

[svc-autorelease]

Released 4.20.0