Update exercises' go.mod to 1.16

[andrerfcsantos]

PR made by running:

gomod-sync.exe update --exercises "..\exercises" --goversion 1.16

See #1954

This must be merged only after exercism/go-test-runner#57

[github-actions]

Dear andrerfcsantos

Thank you for contributing to the Go track on Exercism! 💙
You will see some automated feedback below 🤖. It would be great if you can make sure your PR covers those points. This will save your reviewer some time and your change can be merged quicker.

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_{Automated comment created by PR Commenter 🤖.}

[junedev]

In case someone else is looking for this info: The go version in the go.mod file only affects vendoring (which we don't use) and the use of new language features. You can have go 1.13 installed locally and run a program with a go.mod file that specifies go 1.16 without any problems as long as you don't use any language features that are not available in go 1.13.
https://golang.org/ref/mod#go-mod-file-go

[junedev]

LGTM, feel free to merge

[junedev]

@andrerfcsantos Feel free to change the size label to "medium" here. I feel with all the debugging it was more work than "small".