Order WelcomePageHandlerMapping at lower precedence

This commit orders the `WelcomePageHandlerMapping` at
`Ordered.LOWEST_PRECEDENCE -1` in order to give a chance to other
mappings to handle the incoming requests.

In this case, developers might provide a custom `ViewController` or
custom `HandlerMapping` for the `"/"` path and we should not override
that opinion.

Closes gh-12335

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/WelcomePageHandlerMapping.java

@@ -27,6 +27,7 @@ import org.apache.commons.logging.LogFactory;
 
 import org.springframework.boot.autoconfigure.template.TemplateAvailabilityProviders;
 import org.springframework.context.ApplicationContext;
+import org.springframework.core.Ordered;
 import org.springframework.core.io.Resource;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
@@ -74,7 +75,7 @@ final class WelcomePageHandlerMapping extends AbstractUrlHandlerMapping {
 		ParameterizableViewController controller = new ParameterizableViewController();
 		controller.setViewName(viewName);
 		setRootHandler(controller);
-		setOrder(0);
+		setOrder(Ordered.LOWEST_PRECEDENCE - 1);
 	}
 
 	@Override

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/servlet/WelcomePageHandlerMappingTests.java

@@ -35,6 +35,7 @@ import org.springframework.boot.test.context.runner.WebApplicationContextRunner;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.core.Ordered;
 import org.springframework.core.io.FileSystemResource;
 import org.springframework.core.io.Resource;
 import org.springframework.http.HttpHeaders;
@@ -62,6 +63,15 @@ public class WelcomePageHandlerMappingTests {
 			.withUserConfiguration(HandlerMappingConfiguration.class).withConfiguration(
 					AutoConfigurations.of(PropertyPlaceholderAutoConfiguration.class));
 
+	@Test
+	public void isOrderedAtLowPriority() {
+		this.contextRunner.withUserConfiguration(StaticResourceConfiguration.class)
+				.run((context) -> {
+					WelcomePageHandlerMapping handler = context.getBean(WelcomePageHandlerMapping.class);
+					assertThat(handler.getOrder()).isEqualTo(Ordered.LOWEST_PRECEDENCE - 1);
+				});
+	}
+
 	@Test
 	public void handlesRequestForStaticPageThatAcceptsTextHtml() {
 		this.contextRunner.withUserConfiguration(StaticResourceConfiguration.class)