Make go.mod of external-dependencies declare go 1.16

[andrerfcsantos]

I was testing the external dependency support in one of the exercises and noticed something I wasn't aware: apparently, the Go version declared in the go.mod file affects the indirect dependencies that are downloaded. Since the exercises declare go 1.16 in their go.mod, for parity, the module controlling the dependencies should too.

Here's the problem I encountered:

• I downloaded an exercise
• Made the code use external dependencies
• Added the dependencies to the go.mod file
• Ran go mod tidy to create the go.sum file.
• Ran exercism submit go.sum go..mod <solution_file>
• The test run failed because it was trying to download an indirect dependency not present in the docker image

The workaround for me was:

• Update the Go version of the go.mod of the exercise to match the Go version of the external-dependencies/go.mod file
• Ran go mod tidy to recreate the go.sum file
• Ran exercism submit go.sum go..mod <solution_file>again
• The test run now passed because the go.sum now didn't have any indirect external dependency that wasn't present on the image. All due to the Go version in the go.mod of the exercise now being the same as external-dependencies/go.mod.

This PR changes external-dependencies/go.mod in a way that this workaround of changing the Go version of the exercise's go mod is not necessary.

[coveralls]

Pull Request Test Coverage Report for Build 3324510296

• 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 77.843%

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Totals
Change from base Build 3321664996:	0.0%
Covered Lines:	397
Relevant Lines:	510

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💛 - Coveralls

[ErikSchierboom]

Ping me when a Go maintainer has reviewed this and I'll approve.

[andrerfcsantos]

@ErikSchierboom