Update legal copyright notice on GitHub

[Relequestual]

As per https://github.com/openjs-foundation/cross-project-council/blob/main/governance/IP_POLICY_GUIDANCE.md#2-copyright-notices

[Relequestual]

Blocked. Waiting on OpenJS Foundation legal, per json-schema-org/json-schema-spec#1380 (comment)

[Relequestual]

Closing as per json-schema-org/json-schema-spec#1380 (comment)

After speaking to a number of people, I'm going to close this PR.
Things that are copyrighted, are copyrighted by default. Adding it to the readme doesn't change that.
If it was added, updating it could be problematic.