Update version to 8.7.0

Diff

• https://gitlab.com/gitlab-org/gitlab-qa/-/compare/v8.6.0...26b99b353991c0dc343b25544cc185007b3b4941

Release notes:

New scenarios and scenario updates

• Override report generation flag in update type ... (!1058 - merged)

Checklist before merging

• Diff link is up-to-date.
• Based on the diff, lib/gitlab/qa/version.rb is updated, according to SemVer.
• Release notes are accurate.

Checklist after merging:

• Update the release notes for the newly created tag.