Rename testnet to Kairos

[ian0371]

Proposed changes

Rename testnet to Kairos

Types of changes

• [ ] Bugfix
• [ ] New feature or enhancement
• [x] Others

Checklist

• [x] I have read the CONTRIBUTING GUIDELINES doc
• [x] I have read the CLA and signed by comment I have read the CLA Document and I hereby sign the CLA in first time contribute
• [ ] Lint and unit tests pass locally with my changes ($ make test)
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)
• [ ] Any dependent changes have been merged and published in downstream modules

Related issues

https://github.com/klaytn/klaytn/pull/2184