Resolve "Update copyright year in headers"

This MR deletes the outdated scripts used for managing copyright headers and provides a new one in `scripts/maintainer-tools/copyright-headers`. Also, similarly to before, it also provides a Dockerfile for a `gekkofs/copyright-header` image that encapsulates a version of Osterman's [copyright-header](https://github.com/osterman/copyright-header/) Gem appropriately patched for our needs. Please refer to the README.md file in `scripts/maintainer-tools/copyright-headers` for more details.

Closes #142

See merge request !83