Historical "Incoming traffic" information missing in UI (7.1.0)

## Expected Behavior

The `Incoming traffic` graph used on the System Overview page (`/system/overview`), etc. should either provide a means for the user to view pre-`input-indexed` traffic values or automatically backfill using whatever old data is available.

## Current Behavior

The UI seems to suggest pre-`7.1.0` information was lost (when in fact the graph is displaying something different than before; info still available in the API response, but not ideal).

## Steps to Reproduce (for bugs)

1. Upgrade an environment with history to `7.1.0`
2. Look at the `Incoming traffic` graph and see it only has info as of `7.1.0`

## Context

Checking out `7.1.0`, and specifically trying to better understand the implications of #25501.

## Your Environment

* Graylog Version: `7.1.0`