Maintenance: update Java version and update/remove dependencies

[DCKcode]

I'm using this PR to merge a few maintenance tasks. What this MR does:

• Updates the Java version from the unsupported 15 to Java 17 LTS, where it can stay unchanged for longer. Updates are in the readme as well as the GitHub Action that builds a release.
• Do some plumbing in Gradle to use the latest version there also.
• Updates the few dependencies we have to the latest version.
• Removes a dependency I used for a functional programming construct that I replaced with a home made utility class.

These changes are all lightly intertwined so I figured I'd bundle them in a single PR .

[complexspaces]

This looks good to me. My only nit is that the if( spacing is inconsistent with other places that use if (. I only left one comment but it happens in quite a few places visible in the diff.