insights: note how many dashboards an insight appears on in the delete modal

[Joelkw]

#33637 as a followup, we should note how many dashboards this insight appears on in this modal:

May need very brief design but that does not need to block this – a text sentence is fine too. We already have this functionality on the edit screen so this is likely easy to add:

[sourcegraph-bot-2]

Heads up @Joelkw @felixfbecker @vovakulikov @unclejustin - the "team/code-insights" label was applied to this issue.