Remove Twilio insiders program note since the program has been on hold. Slack: https://twilio.slack.com/archives/CGQPL0RPH/p1652909106203359

Fixes

A short description of what this PR does.

[x] 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.

twilio / terraform-provider-twilio