Drop support for Tomcat 7

[philwebb]

Drop support for Tomcat 7 and remove StoreMergedWebXmlListener.

[htynkn]

hey @philwebb , Tomcat 7 support java 6 and later and Tomcat java 7 and later. If we drop support for tomcat 7, that means if spring boot user choose tomcat and they must use java 7 and later.

I see this task only marked as enhancement without any milestone information, can I ask which spring boot version plan to drop support for tomcat 7?

[snicoll]

@htynkn Spring Boot 2.0 will require java 8 by default (as does Spring 5).