Add property for registering default Servlet

This commit adds a new configuration property for configuring the
registration of the default Servlet in Servlet containers.

`"server.servlet.default-servlet.registered=false"`

The default of this property is still `true`, as it's been the case in
previous releases.

Closes gh-21214

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

@@ -38,6 +38,7 @@ import org.springframework.boot.web.server.Compression;
 import org.springframework.boot.web.server.Http2;
 import org.springframework.boot.web.server.Shutdown;
 import org.springframework.boot.web.server.Ssl;
+import org.springframework.boot.web.servlet.server.DefaultServlet;
 import org.springframework.boot.web.servlet.server.Encoding;
 import org.springframework.boot.web.servlet.server.Jsp;
 import org.springframework.boot.web.servlet.server.Session;
@@ -226,6 +227,9 @@ public class ServerProperties {
 		 */
 		private String applicationDisplayName = "application";
 
+		@NestedConfigurationProperty
+		private final DefaultServlet defaultServlet = new DefaultServlet();
+
 		@NestedConfigurationProperty
 		private final Encoding encoding = new Encoding();
 
@@ -263,6 +267,10 @@ public class ServerProperties {
 			return this.contextParameters;
 		}
 
+		public DefaultServlet getDefaultServlet() {
+			return this.defaultServlet;
+		}
+
 		public Encoding getEncoding() {
 			return this.encoding;
 		}

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

@@ -53,6 +53,8 @@ public class ServletWebServerFactoryCustomizer
 		map.from(this.serverProperties::getAddress).to(factory::setAddress);
 		map.from(this.serverProperties.getServlet()::getContextPath).to(factory::setContextPath);
 		map.from(this.serverProperties.getServlet()::getApplicationDisplayName).to(factory::setDisplayName);
+		map.from(this.serverProperties.getServlet().getDefaultServlet()::isRegistered)
+				.to(factory::setRegisterDefaultServlet);
 		map.from(this.serverProperties.getServlet()::getSession).to(factory::setSession);
 		map.from(this.serverProperties::getSsl).to(factory::setSsl);
 		map.from(this.serverProperties.getServlet()::getJsp).to(factory::setJsp);

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

@@ -74,6 +74,14 @@ class ServletWebServerFactoryCustomizerTests {
 		verify(factory).setDisplayName("TestName");
 	}
 
+	@Test
+	void testCustomizeDefaultServlet() {
+		ConfigurableServletWebServerFactory factory = mock(ConfigurableServletWebServerFactory.class);
+		this.properties.getServlet().getDefaultServlet().setRegistered(true);
+		this.customizer.customize(factory);
+		verify(factory).setRegisterDefaultServlet(true);
+	}
+
 	@Test
 	void testCustomizeSsl() {
 		ConfigurableServletWebServerFactory factory = mock(ConfigurableServletWebServerFactory.class);