Don't impose a gradle 6.7 requirement (#1579)

gradle-baseline-java/src/main/groovy/com/palantir/baseline/tasks/CheckExplicitSourceCompatibilityTask.java

@@ -33,6 +33,7 @@
 import org.gradle.api.specs.Spec;
 import org.gradle.api.tasks.TaskAction;
 import org.gradle.api.tasks.options.Option;
+import org.gradle.util.GradleVersion;
 
 /**
  * By default, Gradle will infer sourceCompat based on whatever JVM is currently being used to evaluate the
@@ -56,6 +57,12 @@ public CheckExplicitSourceCompatibilityTask(ObjectFactory objectFactory) {
         onlyIf(new Spec<Task>() {
             @Override
             public boolean isSatisfiedBy(Task element) {
+                if (GradleVersion.current().compareTo(GradleVersion.version("6.7")) < 0) {
+                    // We're cheekily using this 'getRawSourceCompatibility' method which was only added in Gradle 6.7
+                    // https://github.com/gradle/gradle/commit/8e55abb151e7d933c8348b32b6163fb535254a08.
+                    return false;
+                }
+
                 // sometimes people apply the 'java' plugin to projects that doesn't actually have any java code in it
                 // (e.g. the root project), so if they're not publishing anything, then we don't bother enforcing the
                 // sourceCompat thing. Also they might apply the publishing plugin just to get the 'publish' task.