Downgrade version settings to support flutter 2.8.0+ (#139)

pubspec.yaml

@@ -5,7 +5,7 @@ version: 1.0.1
 homepage: https://livekit.io
 
 environment:
-  sdk: ">=2.17.0 <3.0.0"
+  sdk: ">=2.15.0 <3.0.0"
   flutter: ">=1.17.0"
 
 dependencies:
@@ -13,14 +13,14 @@ dependencies:
     sdk: flutter
   flutter:
     sdk: flutter
-  async: ^2.8.2
-  collection: ^1.16.0
+  async: ^2.6.1
+  collection: ^1.15.0
   fixnum: ^1.0.1
-  meta: ^1.7.0
-  http: ^0.13.4
+  meta: ^1.3.0
+  http: ^0.13.3
   logging: ^1.0.2
   uuid: ^3.0.6
-  synchronized: ^3.0.0+2
+  synchronized: ^3.0.0
   protobuf: ^2.0.1
   flutter_webrtc: ^0.8.12
   dart_webrtc: ^1.0.6