Test against Gradle 6.9.3

Closes gh-32757

Author: wilkinsona

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

@@ -42,7 +42,7 @@ public static List<String> allCompatible() {
 		if (isJava16()) {
 			return Arrays.asList("7.0.2", "7.1", "7.2", "7.3.3", "7.4.2", GradleVersion.current().getVersion());
 		}
-		return Arrays.asList("6.8.3", "6.9.2", "7.0.2", "7.1.1", "7.2", "7.3.3", "7.4.2",
+		return Arrays.asList("6.8.3", "6.9.3", "7.0.2", "7.1.1", "7.2", "7.3.3", "7.4.2",
 				GradleVersion.current().getVersion());
 	}