feat: go 1.20 compatibility

[nmemoto]

Fixes

This PR updates the application to be compatible with go 1.20

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.

[nmemoto]

All tests are green
https://github.com/nmemoto/sendgrid-go/actions/runs/5285866793/jobs/9564752335

[Abonive]

Any update on this? @childish-sambino

[natiiaparicio]

When do you plan to review/approve this PR ? @childish-sambino

[porterctrlz]

What's the blocker to this being merged?

[porterctrlz]

Also runs cleanly on 1.21: https://github.com/controlzee/sendgrid-go/actions/runs/6713922883, but I'll take what I can get in terms of merged PRs

[lopezator]

No maintenance here?

[porterctrlz]

@tiwarishubham635 would you be placed to merge stuff like this?

[matthew-raul]

This is sad.

[aki-s]

It seems @childish-sambino had already retired from Twillio..