updated dependencies/plugins; plus junit updates

[cweedall]

With the update for JUnit, there were some breaking changes "Assert" vs. "Assertions", for example. These changes were done to reflect the dependency upgrades. There were no "assertTrue" methods anymore, so these were updated to failures in most cases. I included the exceptions in the fail() calls, but we could also simply use the original message, if desired.

Also organized the imports in a few places and removed unused ones.

I originally updated the Java version to 17 which is the most recent LTS. However, I opted to keep it at Java 8 for now because I did not know if there was a reason to keep it at that version. If there is no reason not to move to 17, then I can update the README to point to where to download Java 17 (probably Adoptium's releases who have JDKs and "JREs" which are really stripped-down JDKs).

OpenAPI's generator appears to have a Java 11 requirement. It all builds locally fine so I only noticed after pushing to Github. I made updates to reflect the need for Java 11.

[cweedall]

For some reason, there are unit test failures appearing in Github which do not appear locally for me. I will update these after reviewing, so that the builds can succeed.

[mosoriob]

Thanks @cweedall for the pull request.

Update the java version is a good idea.

#174