docs: update witness repo links

[noqcks]

https://github.com/testifysec/witness -> https://github.com/in-toto/witness

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[behnazh-w]

@noqcks Thanks for the PR. We follow conventional commits for our commit messages and that's why one of the CI tests is failing. Can you please change all the commit messages to have a scope, e.g.,:

docs: remove testifysec/witness

Also, you need to follow the instructions to sign the Oracle Contributor Agreement. Please let me know if you face any issues.

[behnazh-w]

Closing this PR because the Witness links are already fixed via #606.