Migrate BWC tests to functional test repository

[joshuarrrr]

Currently, the backwards compatibility (BWC) tests our CI uses are stored in this repo: https://github.com/opensearch-project/OpenSearch-Dashboards/tree/main/cypress

But they're not really easily run-able or debug-able within this project. When the CI runs them, it does so by actually pulling https://github.com/opensearch-project/opensearch-dashboards-functional-test , which is where all our Cypress test runners are actually defined.

It would make more sense for the BWC tests to simply be part of that repository, where we could more easily benefit from shared helpers, functions, test data, etc.

[ashwin-pc]

@joshuarrrr #3728 talks about doing the exact opposite. Should we close this in favor of that discussion?

[joshuarrrr]

Eh, if we're going that direction, we'll still need to update the machinery of BWC tests to match the other cypress tests, so I'd be more inclined to just update this issue to point in that direction.