fix: drop reserved source key in slog.Logger

The `source` key in slog.Logger is a reserved key and should not be used because it causes a panic.
This commit removes the `source` key from the logger.

Note: We already enforce the forbidden keys as per #42049.
However, our lint jobs currently only run on Ubuntu runners, while the code in question compiles for Darwin targets only.

Signed-off-by: Tiago Silva <tiago.silva@goteleport.com>

lib/vnet/dns/osnameservers_darwin.go

@@ -96,6 +96,6 @@ func (s *OSUpstreamNameserverSource) upstreamNameservers(ctx context.Context) ([
 		nameservers = append(nameservers, nameserver)
 	}
 
-	slog.DebugContext(ctx, "Loaded host upstream nameservers.", "nameservers", nameservers, "source", confFilePath)
+	slog.DebugContext(ctx, "Loaded host upstream nameservers.", "nameservers", nameservers, "config_file", confFilePath)
 	return nameservers, nil
 }