remove custom repo
update groupid to match artifact in maven central.
bump version also to that now deployed to maven central.
bump checkstyle version to 7.7 to make compatible.
updated checkstyle.xml to ignore existing issues which are prolific
which are now flagged in latest version as some bugs in previous meant they we'ren't detected e.g. https://github.com/checkstyle/checkstyle/issues/3320
fixing some violations which are not too prolific.
(cherry picked from commit b8ebe0577504fa0b2c39471f36656a0fe0ad59bd)
- added missing license headers found by maven rat plugin.
most of them added automatically via apache-rat
a few manually added
- added apache-rat maven plugin to the build cycle
