docs: standardize license headers

Add license headers clarifying the copyright of source code files.

Checklist

[x] I have read the contributing guidelines and signed the CLA.
[x] I have referenced an issue containing the design document if my change introduces a new feature.
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[x] I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security vulnerability, I confirm that I got approval (please contact security@ory.sh) from the maintainers to push the changes.
[x] I have added tests that prove my fix is effective or that my feature works.
[x] I have added the necessary documentation within the code base (if appropriate).