Bump version to 1.16.0

• Diff: https://gitlab.com/gitlab-org/gitlab-triage/compare/v1.15.0...0c5986cc84f6e0ecaa1ce3c0add84a535f8e8d6f

• Release notes:

### New features and features updates

- !191 Add epics as a resource

### Fixes

- !196 Set 'source' and 'source_id' to nil when '--all-projects' is given

### Other changes (tooling, technical debt)

- !198 Back off from rate limit on GitLab API
- !197 Only clear 'options.source' when '--all-projects' is given

• 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.