Update version to 8.8.0

Diff

https://gitlab.com/gitlab-org/gitlab-qa/compare/v8.7.0...68e4bbfd0e02ffa43833d6c76bb2f0d7d2044193

Release notes

New scenarios and scenario updates

• Add chaos test via toxiproxy (!1040 - merged)

Other changes (CI, backstage)

• Makes sanity job check match on expected output... (!1029 - merged)
• Remove narrowly pinned versions (!1060 - 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.