Bump version to 5.13.6

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v5.13.5...e0eb3e4c2382ee8ff9e2b9620fdf4fd37124236b

• Release notes:

### Fixes
Reverting login-related changes and other potential causes of the recent failure to pull images. Reverts:

- !467 Don't skip pull by default.
- !468 Extract component base class.
- !474 Make sure we log into the registry when the image is private

• Checklist before merging:

  • Diff link is up-to-date.
  • Based on the diff, lib/gitlab/qa/version.rb is updated, according to SemVer.
  • Release notes are accurate.

• Checklist after merging:

  • Create a tag for the new release version.