Switch to jakarta Servlet API

[Conal-Tuohy]

javax.servlet package is unsupported in later versions of some containers (Tomcat 10, Jetty 11), so we need a branch in which we have switched to jakarta.servlet to support those containers as they become the current versions in software package repositories.

[Conal-Tuohy]

A bit of refactoring might be helpful first to separate the differently named APIs into separate Java source code files, so that the differences between the javax and jakarta branches are consolidated in one part of the codebase, isolated from the core program logic.