move the go module vanity url to metallb.io

[fedepaol]

Is your feature request related to a problem?

As part of the migration from metallb.universe.tf to metallb.io, we should rename the modules and setup the redirect of the vanity url.

Describe the solution you'd like

First reference I found is https://blog.jbowen.dev/2020/07/using-go-vanity-urls-with-hugo/
In general, this should not be an issue for metallb per se, as the references are local to the module, but we need to honor it for external projects that may want to consume the API, for example.

Additional context

No response

I've read and agree with the following

• I've checked all open and closed issues and my request is not there.
• I've checked all open and closed pull requests and my request is not there.