Prevent potentially misleading error message about "no heartbeat"

In the context of
github.com/os-autoinst/openQA
we observed error messages coming from Mojo::Server::Prefork
when otherwise there is no harmful effect observed. As the underlying
prefork module should only inform about a potentially erroneous
situation but can not know about the effect on any component that uses
the prefork module the log message instead should not claim an error but
rather only inform. Hence this commit reduces the log level from error
to info accordingly.

Related progress issue: https://progress.opensuse.org/issues/138536

lib/Mojo/Server/Prefork.pm

@@ -110,7 +110,7 @@ sub _manage {
     next unless my $w = $self->{pool}{$pid};
 
     # No heartbeat (graceful stop)
-    $log->error("Worker $pid has no heartbeat ($ht seconds), restarting (see FAQ for more)") and $w->{graceful} = $time
+    $log->info("Worker $pid has no heartbeat ($ht seconds), restarting (see FAQ for more)") and $w->{graceful} = $time
       if !$w->{graceful} && ($w->{time} + $interval + $ht <= $time);
 
     # Graceful stop with timeout