I copied and pasted your code, compiled it, and it worked flawlessly under tomcat. So it's not a code issue.
I'm a little worried that the error makes reference to doEndTag() which looks very much like the container thinks you want to play with classic tags rather than SimpleTagSupport.
Is it possible that somewhere, either in your class compilation or your tomcat setup you are refering to old versions of servlet-api or other core jar files? It just sounds like something is picking up a version of the servlet api prior to 2.4!
Sorry not much help, but it might give you some hints as to where to look!
I have the same problem, but I can't solve it. I am using Tomcat 6 and InteliJ Idea. I cleared all their caches, looked over my dependencies to make sure I have no dependency to an old servlet jar and still I get the error.