Remove legacy "Whitelabel" Terms

[crweiner]

Issue Summary

According to the Docs, SendGrid wishes to move away from the term "Whitelabel" and replace it with other terms, such as "domain authentication", "link branding" and "reverse DNS".

Details:

In order to keep this verbiage consistent, this repo needs to update its terms and replace "Whitelabel" with the more specific terms as described in the Doc.

This was previously accomplished in the SendGrid Python repo here and I am looking to replicate this update to this repo as well.