Bump version to 1.15.0

• Diff: https://gitlab.com/gitlab-org/gitlab-triage/compare/v1.14.3...3f8529aa2025ef0caff41ab49d4cb01dc7652e98

• Release notes:

### New features and feature updates

- !193 Allow Commenting on Summaries.

  Adds a new action `comment_on_summary`

### Fixes

- !192 Handle when the response is an unexpected string.
- !190 Add support to use Project/Group ID when triaging with GraphQL.
- !189 Add responding to ratelimits in GraphQL responses.

• 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.