Store/retrieve tests metadata in/from pages instead of artifacts

This is a "reopening" of #335675 (closed) which we might want to try again now that the issues we had with Pages should be fixed.

Currently, we store and retrieve the Knapsack report, Flaky tests report, and Test mappings in/from artifacts: https://gitlab.com/gitlab-org/gitlab/-/blob/1c737e466e7336e6ca1111e8fe96034126e09b13/scripts/rspec_helpers.sh#L3-70

We may use Pages instead to expose these files for the following reasons:

• We're only interested in the latest state of these files, so using Pages makes sense here
• That would simplify the logic to retrieve these files (currently, we first look for the job ID, and then download its artifacts)
• That would reduce the load on GitLab.com's infrastructure as serving static files from Pages is much cheaper than performing multiple API calls to retrieve the job and artifacts
• That would make https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/943 not needed as requests to Pages don't need to be authenticated (as long as the pages are public)