Bump version to 1.9.0 for first_contribution?

• Diff: https://gitlab.com/gitlab-org/gitlab-triage/compare/v1.8.0...ea716595cd6a29157b44ea505ab9563f97863dcb

• Release notes:

### New feature

- !157 Add Resource::MergeRequest#first_contribution?

• 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.