Remove yaml v1

## What does this MR do?

Replaces`gopkg.in/yaml.v1` with `gopkg.in/yaml.v2`

## Why was this MR needed?

We already vendor in gopkg.in/yaml.v2 and downstreams tend not to have v1 packaged any more.

## Are there points in the code the reviewer needs to double check?

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/blob/master/CHANGELOG.md) entry added
- [ ] Documentation created/updated
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)

## What are the relevant issue numbers?

* Closes #1499 
* Replaces !247

See merge request !307