Bump version to 7.8.1

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v7.8.0...3dca8a8a85aa6e35825cf09a4c9088a3d9cc7315

• Release notes:

### Fixes

- !749 Fix testcase title mismatch bug in reporting script

• 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.