Disable JNDI on Jetty by default

Our Investigations show that JNDI lookup is quite costly on Jetty and it
was enabled by default while embedded tomcat does not. This commit
excludes `jetty-jndi` to align the behavior with Tomcat.

If you are relying on JNDI, the `jetty-jndi` should simply be added in
your project.

Closes gh-4710

spring-boot-starters/spring-boot-starter-jetty/pom.xml

@@ -33,6 +33,12 @@
 		<dependency>
 			<groupId>org.eclipse.jetty.websocket</groupId>
 			<artifactId>javax-websocket-server-impl</artifactId>
+			<exclusions>
+				<exclusion>
+					<groupId>org.eclipse.jetty</groupId>
+					<artifactId>jetty-jndi</artifactId>
+				</exclusion>
+			</exclusions>
 		</dependency>
 	</dependencies>
 </project>