Update Code of Conduct to reference CNCF CoC

[john-h-kastner-aws]

Description of changes

Issue #, if available

Checklist for requesting a review

The change in this PR is (choose one, and delete the other options):

• A breaking change requiring a major version bump to cedar-local-agent (e.g., changes to the signature of an
  existing API).
• A backwards-compatible change requiring a minor version bump to cedar-local-agent (e.g., addition of a new API).
• A bug fix or other functionality change requiring a patch to cedar-local-agent.
• A change "invisible" to users (e.g., documentation, changes to "internal" crates, etc.)
• A change (breaking or otherwise) that only impacts unreleased or experimental code.

I confirm that this PR (choose one, and delete the other options):

• Updates the "Unreleased" section of the CHANGELOG with a description of my change (required for major/minor
  version bumps).
• Does not update the CHANGELOG because my change does not significantly impact released code.

Testing

Add a description on how the code changes were tested, if applicable.

Hint: run ./scripts/build_and_test.sh script to validate your changes locally before submitting a PR.