UP-4209 require Servlet3 / Tomcat 7+.

[UP-4209](https://issues.jasig.org/browse/UP-4209) : Updates `README.md` and release notes documentation to state requirement of Servlet v3 / Tomcat 7 (rather than stating support for Servlet 2.5 which implies Tomcat 6.)

This requirement is only documentation-deep at this point.  This updates the documentation so that the project can confidently not worry about Servlet 2.5 / Tomcat 6 support for uPortal 4.2 and beyond.  Presumably future opportunities will arise to do servlet-spec-y things that benefit from that Servlet 3 version.