Spring boot lib update

[richardiw]

At my own interview a few months ago, and while interviewing today, it's noticeable that the Spring version is very out of date (i.e obsolete for several years)
Also, when we ask candidates about testing, there is no boiler plate setup with test dependencies which means that it's difficult to actually have time to write a test. It's easy for a a candidate to say they are keen on testing, and say 'of course, I would write tests if I had the time' but we should make it as easy as possible for candidates to demo this skill.

This PR:

• updates Spring and Gradle
• adds a test class with placeholder test. This is similar to what is created by modern version of spring-boot-starter-web anyway.
• now allows Java 17 to be used at runtime.