Bump version to 6.19.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.18.2...e863af7427acf2bb54c7ce1effcea8f496875db2

• Release notes:

### New scenarios and scenario updates

- !629 Add updated mTLS scenario.

### Fixes

- !633 Fix initial password method invocation.

• 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:

  • Update the release notes for the newly created tag.