Make the Servlet spec version configurable

[earldouglas]

Users might be on one of a long list of Servlet versions:

• https://tomcat.apache.org/whichversion.html
• https://jetty.org/download.html#version-history

Currently, the runner uses Tomcat 10.1 (via com.heroku:webapp-runner:10.1.28.0) to support Servlet 6.0.

Let's make this easier to configure, with a setting like servletSpec := "6.0" which then chooses the appropriate runner.