Deleted fleet reference should not render links

[kroepke]

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 (<id>) 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?