Setting up index set when no warm tier is configured is hard

## Expected Behavior

When no warm tier is configured, creating an index set should default to other means.

## Current Behavior

When I have no warm tier configured and the user wants to create an index set, the flow requires the user to select all kinds of warm tier options until the final click for creating the index set fails with a temporary toaster notification:


## Possible Solution

Either:
- Default to other/non-warm-tier/legacy options when no warm tier is configured
- Handle this in form validation to show an error in the form instead of a toaster notification that results from an API request

## Steps to Reproduce (for bugs)

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## Context

## Your Environment

* Graylog Version:
* Java Version:
* OpenSearch Version:
* MongoDB Version:
* Operating System:
* Browser version:

## Checklist
[] This issue fix need to be backported.
[] Does this issue have **security** implications?