Deleted fleet reference should not render links

## Expected Behavior
If a collector references a deleted fleet, it should not try to link to it.
Ideally references should clearly tell the user that the fleet no longer exists, instead of simply showing an ObjectId.

## Current Behavior
We render a dead link.

## Possible Solution
Render placeholder text like `Fleet removed ()` instead of the link.

## Steps to Reproduce (for bugs)
1. Delete the fleet of an active collector
2. Look at the collector instance table.

## 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?

No file or test is named. Start at the collector instance table, reproduce the issue by deleting a fleet referenced by an active collector, and trace how the fleet reference is rendered. Done means deleted fleets no longer produce links and instead clearly identify the fleet as removed, including its id.

Written by the indexing model from the issue text.