This PR revamps e2e tests, while adding a new one for the interactive
demo guide:
- Bumps Cypress from `9.7.0` to `12.11.0`;
- Bumps Cypress GH action from `v2` to `v5`;
- Makes any adjustments needed;
- Fixes a lot of issues identified with existing tests;
- Adds new `demo.spec.ts` e2e test that covers the entire demo guide
flow;
**Note:** Currently does not include `demo.spec.ts` in the GH action, as
it
[fails](https://github.com/Unleash/unleash/actions/runs/4896839575/jobs/8744137231?pr=3656)
on step 2.13 (last step of "user-specific" topic). It runs perfectly
fine locally, though.
Might be placebo, but in general tests seem less flaky now and they may
even be faster (especially when not adding the `demo` one, which would
always take a long time).
