[SR] Policy wizard steps are not disabled when form is invalid

[alisonelizabeth]

Steps to reproduce:

1. Navigate to the Polices tab in Snapshot & Restore and create a policy.

2. Proceed to step 2 (Snapshot settings).

3. Select an index from the list of indices, then deselect it. A field error should display and the "Next" button should be disabled. Note step 1 (Logistics) is still enabled.

4. Navigate back to step 1 (Logistics).

5. Note the Next button is still disabled and the user is stuck unless the browser is refreshed.

Expected behavior:
I would expect the previous steps to also be disabled if there is a field error.

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)

[alisonelizabeth]

This also occurs if you have an invalid field on the retention step (e.g., min count greater than max count) and go back to a previous step without fixing it.