Stop testing Gradle plugin against Gradle 7.6 RCs

Closes gh-33342

Author: wilkinsona

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

@@ -34,17 +34,16 @@ private GradleVersions() {
 
 	public static List<String> allCompatible() {
 		if (isJava18()) {
-			return Arrays.asList("7.3.3", "7.4.2", GradleVersion.current().getVersion(), "7.6-rc-3");
+			return Arrays.asList("7.3.3", "7.4.2", GradleVersion.current().getVersion());
 		}
 		if (isJava17()) {
-			return Arrays.asList("7.2", "7.3.3", "7.4.2", GradleVersion.current().getVersion(), "7.6-rc-3");
+			return Arrays.asList("7.2", "7.3.3", "7.4.2", GradleVersion.current().getVersion());
 		}
 		if (isJava16()) {
-			return Arrays.asList("7.0.2", "7.1", "7.2", "7.3.3", "7.4.2", GradleVersion.current().getVersion(),
-					"7.6-rc-3");
+			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", "6.9.3", "7.0.2", "7.1.1", "7.2", "7.3.3", "7.4.2",
-				GradleVersion.current().getVersion(), "7.6-rc-3");
+				GradleVersion.current().getVersion());
 	}
 
 	public static String minimumCompatible() {