Reserve layer names for future use

Update layered jar support so that the name `ext` and any name
starting `springboot` are reserved.

See gh-19767

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/Layer.java

@@ -41,6 +41,8 @@ public class Layer {
 	public Layer(String name) {
 		Assert.hasText(name, "Name must not be empty");
 		Assert.isTrue(PATTERN.matcher(name).matches(), "Malformed layer name '" + name + "'");
+		Assert.isTrue(!name.equalsIgnoreCase("ext") && !name.toLowerCase().startsWith("springboot"),
+				"Layer name '" + name + "' is reserved");
 		this.name = name;
 	}