Add go1.16 support

[ldez]

• drop go1.13
• add go1.16

We have to wait for the new alpine Docker image for go1.16 before merging the PR.
docker-library/official-images#9639

Also, we will have to update the CI check requirements settings in GitHub

Fixes #1725

[ldez]

The error message returned by govet has been changed, so I don't know how we will handle that to be able to run the tests on go1.14 and go1.15

fixed

[ldez]

@ernado could you update the CI check requirements settings in GitHub?

[ernado]

Hey @ldez, I've updated settings, now it is required to pass go1.{14,15,16} and the go1.13 requirement was dropped.

[sayboras]

maybe it's time to cut off a new release as well ? I have been building from source for ttime being :(

[sayboras]

The related changes in goreleaser goreleaser/goreleaser#1956 and https://github.com/goreleaser/goreleaser/releases/tag/v0.156.1.

Seems like it will just work without any update required for .goreleaser.yaml

[ldez]

@sayboras I agree, can we merge #1739 before?