Bump to version 1.20.0

• Diff: https://gitlab.com/gitlab-org/gitlab-triage/compare/v1.19.0...ce3649f519ecb2fa2f273307f2499efbb030091f

• Release notes:

### New features and features updates

- !213 Add support for `forbidden_labels` in GraphQL engine.

### Fixes

- !215 Handle unexpected response for post_api as well.

• 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.