kv: improve observability into under-replicated ranges

_Adapted from CRDB-40230._

Currently, our logs will only indicate there are under-replicated ranges, but they don't say _why_ these ranges are under-replicated.

The _why_ requires knowledge and analysis of log files (or files in a debug.zip). These are often difficult to parse.

Improve our logging and how we display and report on under-replicated ranges in the DB console.

Jira issue: CRDB-41374

Start by reviewing the existing logging and DB console reporting for under-replicated ranges. Determine how the current system identifies these ranges and what information is needed to explain why they are under-replicated; done means the cause is surfaced in logs and displayed in the DB console.

Written by the indexing model from the issue text.