[contact-importer] Update to support @sendgrid/client v6.X

[aslafy-z]

Closes #427

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Upgrade contact-importer to use @sendgrid/client v6.X

[aslafy-z]

Still need to upgrade Bottleneck to v2. https://github.com/SGrondin/bottleneck#upgrading-to-v2

[thinkingserious]

Hello @aslafy-z,

Thanks again for the PR!

It's HACKTOBERFEST! We want to show our appreciation by sending you some special Hacktoberfest swag. If you have not already, could you please fill out this form so we can send it to you? Thanks!

Team SendGrid DX

[thinkingserious]

Hello @aslafy-z,

Is this still WIP?

Thanks!

With Best Regards,

Elmer

[aslafy-z]

Bottleneck can be upgraded in another PR. It's not WIP anymore @thinkingserious.

[reedsa]

LGTM, thanks!