Update support go version to 1.16+

[ajgajg1134]

dd-trace-go now supports only the 3 latest go versions which is currently 1.16-1.18

What does this PR do?

Motivation

Additional Notes

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Reviewer checklist

• Review the changed files.
• Review the URLs listed in the Preview section.
• Check images for PII
• Review any mentions of "Contact Datadog support" for internal support documentation.

[github-actions]

Preview links (active after the build_preview check completes):
https://docs-staging.datadoghq.com/andrew.glaude/update-go-supported/tracing/setup_overview/setup/go

[katiehockman]

Is there documentation somewhere on what we have to update when we upgrade our version of Go every 6 months? (Assuming that there is something we have to do there)
It would be nice to include this doc as something that needs to be regularly updated.

[ajgajg1134]

Is there documentation somewhere on what we have to update when we upgrade our version of Go every 6 months? (Assuming that there is something we have to do there) It would be nice to include this doc as something that needs to be regularly updated.

Good idea, I wasn't sure quite where to create the docs, so to start I've added a comment within the dd-trace-go README so anyone updating the documentation within our repository will be reminded to update the public docs as well: DataDog/dd-trace-go#1353