Bump version to 1.19.0

• Diff: https://gitlab.com/gitlab-org/gitlab-triage/compare/v1.18.0...259980cfa2e4f7bae19a16cd0dd1bafd11e3e29b

• Release notes:

### Fixes

- !211 Use latest label events to pick issuable labels

• Checklist before merging:

  • Diff link is up-to-date.
  • Based on the diff, lib/gitlab/triage/version.rb is updated, according to SemVer.
  • Release notes are accurate.

• Checklist after merging:

  • Update the release notes for the newly created tag.