Add copyright header workflow

[spwoodcock]

• Most of our repos have a copyright notice at the top of all files.
• Adding and updating these manually is tedious.
• Use Hashicorp Copywrite to create a workflow that does this manually - both adding and updating the dates.
• Add a manual trigger only, so this can be run occasionally.

[spwoodcock]

1. "specifying the year, or range of years, in a license or copyright header isn't a requirement".

2. This is available if needed https://github.com/FantasticFiasco/action-update-license-year

The dates should simply be removed from copyright headers we have, on all repos.

Closing.