Bump VERSION to 1.18.0

• Diff: https://gitlab.com/gitlab-org/gitlab-triage/compare/v1.17.0...5877dbb

• Release notes:

### New features and features updates

- !209 Set custom User-Agent string.

### Fixes

- !207 Don't allow Epics triaging for projects.
- !208 Make sure we're giving title from GraphQL query.

• 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.