Update and release gitlab-experiment for version 0.6.1

• Diff: https://gitlab.com/gitlab-org/gitlab-experiment/-/compare/v0.5.4...v0.6.1

• Release notes:

Includes a new middleware layer for link tracking, and configuration changes for context key generation.

• Checklist before merging:

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

• Checklist after merging:

  • Create a tag for the new release version.