Fix GradleVersion forced-upgrade for minimum required JRE.

Author: nedtwigg

plugin-gradle/src/test/java/com/diffplug/gradle/spotless/GradleIntegrationHarness.java

@@ -29,6 +29,7 @@
 import org.gradle.testkit.runner.BuildTask;
 import org.gradle.testkit.runner.GradleRunner;
 import org.gradle.testkit.runner.TaskOutcome;
+import org.gradle.util.GradleVersion;
 import org.junit.jupiter.api.BeforeEach;
 
 import com.diffplug.common.base.Errors;
@@ -50,27 +51,35 @@ public enum GradleVersionSupport {
 		final String version;
 
 		GradleVersionSupport(String version) {
+			String minVersionForRunningJRE;
 			switch (Jvm.version()) {
 			case 20:
 			case 19:
-			case 18:
 				// TODO: https://docs.gradle.org/current/userguide/compatibility.html
+			case 18:
+				minVersionForRunningJRE = "7.5";
+				break;
 			case 17:
-				this.version = "7.3";
+				minVersionForRunningJRE = "7.3";
 				break;
 			case 16:
-				this.version = "7.0";
+				minVersionForRunningJRE = "7.0";
 				break;
 			case 15:
-				this.version = "6.7";
+				minVersionForRunningJRE = "6.7";
 				break;
 			case 14:
-				this.version = "6.3";
+				minVersionForRunningJRE = "6.3";
 				break;
 			default:
-				this.version = version;
+				minVersionForRunningJRE = null;
 				break;
 			}
+			if (minVersionForRunningJRE != null && GradleVersion.version(minVersionForRunningJRE).compareTo(GradleVersion.version(version)) > 0) {
+				this.version = minVersionForRunningJRE;
+			} else {
+				this.version = version;
+			}
 		}
 	}