De-quarantine multi project pipelines spec

What does this MR do?

To de-quarantine multi-project_pipelines_spec.rb

Reason: This test has been passing in nightly for a week now

• https://gitlab.com/gitlab-org/quality/nightly/-/jobs/442297241#L7623
• https://gitlab.com/gitlab-org/quality/nightly/-/jobs/440658967#L7613
• https://gitlab.com/gitlab-org/quality/nightly/-/jobs/439206809#L7356
• https://gitlab.com/gitlab-org/quality/nightly/-/jobs/438752005#L7580
• https://gitlab.com/gitlab-org/quality/nightly/-/jobs/446870756#L7308
• https://gitlab.com/gitlab-org/quality/nightly/-/jobs/446689764#L7513
• https://gitlab.com/gitlab-org/quality/nightly/-/jobs/445237869#L7400