Bump version to 4.19.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v4.18.0...723ed8e3fa67db4fb66a4d1fc236f46a1cf2719f

• Release notes:

### New scenarios and scenario updates

- !379 Removes the SAML component from GroupSAML.

### Other changes (CI, backstage)

- !375 Use new gitlab-qa-ruby-2.6 version.

• 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.