Upgrade to Tomcat 11.0

This commit upgrades the baseline to Tomcat 11.0 and adapts to the
following behavior changes in Tomcat:

* the MimeHeaders#clear method has been removed
* expired cookies do not set "Max-Age=0" anymore
* responses to HEAD requests do not write the "Content-Length" header
  anymore.

Closes gh-33916

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestMappingIntegrationTests.java

@@ -93,7 +93,6 @@ class RequestMappingIntegrationTests extends AbstractRequestMappingIntegrationTe
 		HttpHeaders headers = getRestTemplate().headForHeaders(url);
 		String contentType = headers.getFirst("Content-Type");
 		assertThat(contentType).isNotNull();
-		assertThat(headers.getContentLength()).isEqualTo(3);
 	}
 
 	@ParameterizedHttpServerTest