Add protobuf 6.x compatibility

[xander1421]

Problem

The current protobuf version constraint (^4.2.0) prevents using livekit_client with protobuf 6.x. This blocks projects that depend on other packages requiring protobuf 6.x.

Breaking changes in protobuf 6.x

1. PbList constructor is now private - createRepeated() methods in generated code use $pb.PbList<T>() which no longer works
2. toProto3Json/fromProto3Json removed from BuilderInfo - These parameters in BuilderInfo constructors were removed

Suggested fix

1. Update pubspec.yaml protobuf constraint to >=4.2.0 <7.0.0
2. Regenerate proto files with a protobuf 6.x-compatible protoc-gen-dart
   • Replace $pb.PbList<T>() with $core.List<T> in createRepeated() methods
   • Remove toProto3Json/fromProto3Json parameters from BuilderInfo constructors

Environment

• Flutter 3.x
• protobuf 6.0.0
• Dart 3.x

[hiroshihorie]

#957 has been merged, thank you for your PR