Update version to 7.26.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v7.25.1...166443039df8066fc906a5f19ec57e2203879182

Release notes

New scenarios and scenario updates

• Add CI Decomposition Omnibus Configuration (!918 - merged)
• Implement create admin token for root user in G... (!926 - merged)
• Allow mounting knapsack report from host (!934 - merged)

Doc changes

• Fix spec failure (!930 - merged)

Other changes (CI, backstage)

• Add screenshots to failure issues (!908 - merged)
• List source of Default templates (!931 - 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.