Drop support for 1.16.x, add support for latest versions of Go

[adityasaky]

Looks like we've missed updating Go versions in our CI, so fast tracking for now all the way through 1.20.x. I imagine we can drop 1.17.x and possibly 1.18.x in the not too distant future.

[adityasaky]

FWIW, I believe #40 is failing because of 1.16.x.

[coveralls]

Pull Request Test Coverage Report for Build 4658040683

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 91.632%

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Totals
Change from base Build 4438785512:	0.0%
Covered Lines:	219
Relevant Lines:	239

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💛 - Coveralls