chore: fix OSPO compliance issues

[minhaz]

This automated PR addresses OSPO compliance requirements for the bigquery-information-schema block repository.

Changes included:

• Appended standard Google Open Source disclaimer to README.md.
• Replaced any legacy or incorrect license file with the standardized LICENSE file (containing verbatim Google Cloud Terms of Service) matching the repository's creation year from git history.
• Added the standardized CONTRIBUTING.md file with CLA links.

[gemini-code-assist]

Code Review

This pull request updates the repository's documentation and licensing files. Specifically, it updates CONTRIBUTING.md with guidelines on how to contribute, changes the LICENSE file to Google Cloud Terms of Service, and adds a disclaimer to README.md regarding the Google Open Source Software Vulnerability Rewards Program. The review feedback correctly points out a contradiction in CONTRIBUTING.md, where instructions on how to contribute are added while still stating that the project does not accept contributions.

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[gemini-code-assist]

There is a direct contradiction in this file. The newly added # How to contribute header and the subsequent sections about signing the CLA and the contribution process (lines 5-33) conflict with line 3, which states THIS PROJECT DOES NOT ACCEPT PATCHES OR CONTRIBUTIONS..

If this repository is open to external contributions, please remove the line stating that contributions are not accepted. If it remains closed to contributions, please revert these additions or use the appropriate non-contribution template.