Decouple index and pipeline dates in rank tests

[StepanBrychta]

At the moment, the rank test code assumes that pipeline dates and index dates always match. However, this won't always be the case going forward, since we decoupled the two dates in the pipeline Terraform stack.

We should make the necessary changes to allow rank tests to read the pipeline date of the current production/staging cluster, possibly by modifying this JSON file.

[!IMPORTANT]
As part of this work, hardcoded rank test pipeline dates introduced in this PR should also be removed.

[pollecuttn]

Reviewer: @paul-butcher

[agnesgaroux]

See HOTFIX: snapshot reporter catalogue-api#874 for where to get the pipeline date now