Bump required Go version to 1.13

[invidian]

1.12 is now EOL for almost 2 years: https://endoflife.date/go.

As a reference, supported Ubuntu versions use either 1.10 or 1.13:
https://packages.ubuntu.com/search?keywords=golang-go

So I think this bump is reasonable.

This is done to get access to stdlib methods like errors.Is and
formatting statement %w, which is de facto standard for handling errors
nowadays.

[lucab]

I think this effectively means moving the module to v23, i.e. a major breaking change.
I'm not fully against it, but it is going to introduce a huge churn across all consumers.

[invidian]

Is there some roadmap for v23 available? What should go in and what not? Or is there some dev branch for v23 available to submit PRs against?