chore: update sdk to rc21

[kpchoy]

Submission Checklist

• [ ] The README.md reflects new features
• [ ] The CHANGELOG.md reflects any feature, bug fixes, or known issues made in the source code
• [ ] New unit tests and integration tests have beed added
• [ ] Code coverage is more or equal to the previous coverage
• [ ] A visual inspection of the Files changed tab was made prior to submitting the pull request ensuring the style guide was followed
• [ ] CI pipeline is green
• [ ] Included screenshot if necessary

Description

https://issues.corp.twilio.com/browse/VBLOCKS-2754

For testing of CallMessage feature in iOS and Android

Contributing to Twilio

All third-party contributors acknowledge that any contributions they provide will be made under the same open-source license that the open-source project is provided under.

• [x] I acknowledge that all my contributions will be made under the project's license.