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This MR deletes the outdated scripts used for managing copyright headers and provides a new one in scripts/maintainer-tools/copyright-headers.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 Gem appropriately patched for our needs. Please refer to the README.md file in scripts/maintainer-tools/copyright-headers for more details.

Closes #142 (closed)