journactl: show info about journal range only at debug level (#21775)

The message that the "journal begins … ends …" has been always confusing to
users. (Before b91ae210 it was "logs begin … end …" which was arguably even
more confusing, but really the change in b91ae210 didn't substantially change
this.)

When the range shown is limited (by -e, -f, --since, or other options), it
doesn't really matter to the user what the oldest entries are, since they are
purposefully limiting the range. In fact, if we are showing the last few
entries with -e or -f, knowing that many months the oldest entries have is
completely useless.

And when such options are *not* used, the first entry generally corresponds to
the beginning of the range shown, and the last entry corresponds to the end of
that range. So again, it's not particularly useful, except when debugging
journalctl or such. Let's just treat it as a debug message.

Fixes #21491.