Bump version to 5.12.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v5.10.1...1ea430ecce9e118eed726b2ee1590f686349440f

• Release notes:

### New scenarios and scenario updates

- !451 Add a new Scenario and component for Jira.

• 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.