Rename property for registering default Servlet

As a follow-up for gh-21214, this commit renames the property to:

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

Closes gh-21214

Author: bclozel

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

@@ -38,7 +38,6 @@
 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;
@@ -227,8 +226,10 @@ public static class Servlet {
 		 */
 		private String applicationDisplayName = "application";
 
-		@NestedConfigurationProperty
-		private final DefaultServlet defaultServlet = new DefaultServlet();
+		/**
+		 * Whether to register the default Servlet with the container.
+		 */
+		private boolean registerDefaultServlet = true;
 
 		@NestedConfigurationProperty
 		private final Encoding encoding = new Encoding();
@@ -263,12 +264,16 @@ public void setApplicationDisplayName(String displayName) {
 			this.applicationDisplayName = displayName;
 		}
 
-		public Map<String, String> getContextParameters() {
-			return this.contextParameters;
+		public boolean isRegisterDefaultServlet() {
+			return this.registerDefaultServlet;
 		}
 
-		public DefaultServlet getDefaultServlet() {
-			return this.defaultServlet;
+		public void setRegisterDefaultServlet(boolean registerDefaultServlet) {
+			this.registerDefaultServlet = registerDefaultServlet;
+		}
+
+		public Map<String, String> getContextParameters() {
+			return this.contextParameters;
 		}
 
 		public Encoding getEncoding() {

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

@@ -53,8 +53,7 @@ public void customize(ConfigurableServletWebServerFactory factory) {
 		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()::isRegisterDefaultServlet).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

@@ -77,9 +77,9 @@ void testCustomizeDisplayName() {
 	@Test
 	void testCustomizeDefaultServlet() {
 		ConfigurableServletWebServerFactory factory = mock(ConfigurableServletWebServerFactory.class);
-		this.properties.getServlet().getDefaultServlet().setRegistered(true);
+		this.properties.getServlet().setRegisterDefaultServlet(false);
 		this.customizer.customize(factory);
-		verify(factory).setRegisterDefaultServlet(true);
+		verify(factory).setRegisterDefaultServlet(false);
 	}
 
 	@Test