feat: Added steps to install RC version and updated major version

[AsabuHere]

Closes

DII-563Generate twilio-csharp into RC branch
A short description of what this PR does.

• Updated major version in build configuration file with RC version
• Added steps in readme file to install RC version
• Deleting Rest folder inside test folder

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

If you have questions, please file a support ticket, or create a GitHub Issue in this repository.

[childish-sambino]

Note that you'll need a similar change for the rc.1 release: twilio/twilio-java@5a34a47