[chore] Improve copyright header handling (#1608)

* [chore] Remove years from all license headers

Years or year ranges aren't required in license headers. Many projects
have removed them in recent years and it avoids a bit of yearly toil.

In many cases our copyright claim was also a bit dodgy since we added
the 2021-2023 header to files created after 2021 but you can't claim
copyright into the past that way.

* [chore] Add license header check

This ensures a license header is always added to any new file. This
avoids maintainers/reviewers needing to remember to check for and ask
for it in case a contribution doesn't include it.

* [chore] Add missing license headers

* [chore] Further updates to license header

* Use the more common // indentend comment format
* Remove the hack we had for the linter now that we use the // format
* Add SPDX license identifier

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