Update Confluent Cloud banner UTM campaign parameter

[viveksnh]

Summary

• Updates the UTM campaign parameter in the Confluent Cloud banner
• Changes from utm_campaign=oss-repos to utm_campaign=tm.plg.cflt-oss-repos

Context

The original banner PR was merged with the old UTM parameter. This PR updates it to the correct tracking parameter.

Changes

• Updated UTM campaign parameter in README.md banner link

Testing

• Banner link works correctly with new UTM parameter
• No other changes to the README

[confluent-cla-assistant]

🎉 All Contributor License Agreements have been signed. Ready to merge.
_{Please push an empty commit if you would like to re-run the checks to verify CLA status for all contributors.}