Update to flutter-2.8.0 (#68)

Updated to the flutter 2.8.0. See also:
 - https://github.com/sony/flutter-embedded-linux/releases/tag/40a99c5951
 - https://github.com/flutter/flutter/tree/2.8.0

lib/elinux_remote_devices_config.dart

@@ -77,7 +77,7 @@ class ELinuxRemoteDevicesConfig {
     for (final MapEntry<int, dynamic> entry in typedList.asMap().entries) {
       try {
         revived.add(ELinuxRemoteDeviceConfig.fromJson(entry.value));
-      } on ELinuxRemoteDeviceRevivalException catch (e) {
+      } on ELinuxRemoteDeviceRevivalException catch (_) {
         // TODO(hidenori): Corrensponds to the format difference
         // from the default schema and uncooment here.
         //