util/log: use v1 log parser in case it is undefined

[aliher1911]

Previously merge-logs were failing to read file if it doesn't contain
log format config string and no --format command line option was
specified.
This is not convenient as most of logs that require merging are still
from older versions of cockroach. And for newer versions we should be
able to use config string to autodetect.
This diff changes the behaviour to use crdb-v1 format by default.

Release note: None

Fixes #68278

[cockroach-teamcity]

This change is 

[knz]

There are two different issues at hand here.

merge-logs were failing to read file if it doesn't contain log format config string and no --format command line option was specified.

The code is meant to auto-detect the format from the information present at the start of log files. Can you show us what example log files you've tried to process which caused an error? I'd rather fix the auto-detection first before introducing a fallback.

Then, separately, you mentioned a "panic" to me before. Regardless of whether there is a fallback, we should get the error checking right. Even without a fallback, there should be no panic. Have you identified where the panic comes from?

[knz]

oh I see the linked issue has some repro steps. let's look at this first

[aliher1911]

Fixed by a more consistent change.