Bump version to 6.3.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.2.0...88e792afd8b03ab086411efedee242039b9d729d

• Release notes:

Other changes (CI, backstage)

• !533 (merged) Extract ReportAsIssue and GitlabIssueClient

• !535 (merged) Pull QA image before building specs container

• 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.