Enable Gradle plugin tests on Java 20

Closes gh-35931

Author: scottfrederick

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/build.gradle

@@ -136,5 +136,5 @@ artifacts {
 }
 
 toolchain {
-	maximumCompatibleJavaVersion = JavaLanguageVersion.of(18)
+	maximumCompatibleJavaVersion = JavaLanguageVersion.of(20)
 }

spring-boot-project/spring-boot-tools/spring-boot-gradle-test-support/src/main/java/org/springframework/boot/testsupport/gradle/testkit/GradleVersions.java

@@ -19,6 +19,7 @@
 import java.util.Arrays;
 import java.util.List;
 
+import org.gradle.api.JavaVersion;
 import org.gradle.util.GradleVersion;
 
 /**
@@ -31,12 +32,20 @@ public final class GradleVersions {
 	private GradleVersions() {
 	}
 
+	@SuppressWarnings("UnstableApiUsage")
 	public static List<String> allCompatible() {
+		if (isJavaVersion(JavaVersion.VERSION_20)) {
+			return Arrays.asList("8.1.1", "8.2-rc-1");
+		}
 		return Arrays.asList("7.5.1", GradleVersion.current().getVersion(), "8.0.2", "8.1.1", "8.2-rc-1");
 	}
 
 	public static String minimumCompatible() {
 		return allCompatible().get(0);
 	}
 
+	private static boolean isJavaVersion(JavaVersion version) {
+		return JavaVersion.current().isCompatibleWith(version);
+	}
+
 }