Rewrite checkstyle script

[flyingsilverfin]

Goal

We want to update our Checkstyle tool to validate that the copyright headers match the following string for the date: 2015 - present. Updating this across all the license types will enforce an update in all the repositories across the organisation.

Note: we still want to have the tool be kotlin/java based, and ideally throw 0 null pointers! This may require implementing the checkstyle regex ourselves... Note that we had an intern work on a rust-based version here: https://github.com/Teimurazi-Toloraia/copyright-script/tree/main

A simple version will do for now, although if we can add more 'types' of checkstyle to our own tool that would be ideal, such as having a 1) CopyrightChecker, 2) NewlineChecker (checks that end of each file has a newline) etc.

[flyingsilverfin]

Would close #420

[farost]

@flyingsilverfin we need to update the description or close the whole issue(s) following the new realities!