Describe the problem

When parsing logs from 20.2 format detector will panic unhelpfully. It requires --format crdb-v1 to be passed to proceed which is unfriendly.

To Reproduce

Try

cockroach debug merge-logs with logs 

created by cockroach-20.2 and observe

ERROR: decoding format: failed to extract log file format from the log

Expected behavior
We should stick to v1 format if detector fails maybe or provide some helpful message at least?

This change was introduced in #65633