Sync notice and year

[leoll2]

Copyright notices which include a year, e.g. Copyright 2021 Acme Co., should be updated when the year changes.¹

This pre-commit could support it in various ways:

1. Check if the template contains a year which is not the current one
2. Test the source files against a variable-year notice template and flag those files whose year doesn't match the actual one.

¹ Actually, depending on the laws, this is not strictly required after the software has been released. It's still useful while the software is being developed, though.