[chore] License headers are inconsistent

[dmitryax]

Some license headers have a year in the copyright title (mostly outdated 2019 or 2020), and some of them don't. I remember we agreed to remove the year from the header and use // Copyright The OpenTelemetry Authors across the codebase. We should update all of the files and enforce it in the CI.

[codeboten]

Now that #21384 is closed, make checklicense enforces the consistency of the license