[Upgrade Assistant] Investigate credential hash

[alisonelizabeth]

In #123727, we made changes to how the credential hash is created in Upgrade Assistant to exclude reindexOptions.

As a follow-up, we should consider whether it would be a better approach to update the credential hash with the new reindexOptions attributes, rather than exclude them.

Related work that likely caused the original issue: #58890, #58890.

[elasticmachine]

Pinging @elastic/platform-deployment-management (Team:Deployment Management)

[sebelga]

I think it would be better to only include in the hash what we know is unique and necessary for the functionality. Adding the full attributes object, including the errorMessage string does not seem robust.

By defining what we need and be explicit about it in the code we will prevent future regressions.