Bump version to 5.13.5

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v5.13.4...2d1c475ce343a26fdc912a975cba100bbbe114f2

• Release notes:

### Fixes

- !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.