Update version to 9.1.1

Diff

v9.1.0...fc81395e

Release notes:

Fixes

• Add parallel gem as runtime dependency (!1134 - 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.