docs: update copyright headers to 2020-2026 range

[pranavs321]

What kind of change does this PR introduce?

This is a documentation and automation update to standardize copyright headers.

What is the current behavior?

Currently, copyright headers are inconsistent across the project, with various files showing different years (e.g., 2021, 2023, 2024). The automation in main.yml did not enforce a specific year range, leading to manual updates and potential CI failures.
What is the new behavior?

I have updated the addlicense command in .github/workflows/main.yml to include the -y 2020-2026 flag. I also ran the tool locally to apply this range to all relevant files, ensuring project-wide consistency for the current year.
Which issue(s) this PR fixes

Fixes #1534

[spencerschrock]

Hi, the guidance in #1534 seems to say different, specifically that we should omit a years field so that we dont need to update it:

Instead, many LF project communities recommend using a more general statement in a form similar to the following (where XYZ is the project’s name):

Copyright The XYZ Authors.
Copyright The XYZ Contributors.
Copyright Contributors to the XYZ project.
These statements are intended to communicate the following:
the work is copyrighted;
the contributors of the code licensed it, but retain ownership of their copyrights; and
it was licensed for distribution as part of the named project.
By using a common format, the project avoids having to maintain lists of names of the authors or copyright holders, years or ranges of years, and variations on the (c) symbol.

https://www.linuxfoundation.org/blog/blog/copyright-notices-in-open-source-software-projects

So if we're going to touch the copywrite in every file, can we do it with an empty -y="" argument?

P.S. How did you get the tool to re-create all the licenses locally? When I tried it just now, it skipped all the files because they already had a license declared

[github-actions]

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