update vm_service_protos package to version 2 (#9540)

Update to vm_service_protos version ^2.0.0, this allows us to update protobuf from 3.1.0 -> 4.2.0 (this is only a transitive dep).

Should unlock the fix for https://github.com/flutter/devtools/issues/9534, although we still need a new version of `vm_service_protos` to be published which allows `protobuf` ^5.0.0 (we need 5.1.0 to get the default size increase).

packages/devtools_app/pubspec.yaml

diff --git a/packages/devtools_app/pubspec.yaml b/packages/devtools_app/pubspec.yaml
index 939f4a4..30773bf 100644
--- a/packages/devtools_app/pubspec.yaml
+++ b/packages/devtools_app/pubspec.yaml
@@ -54,7 +54,7 @@
   string_scanner: ^1.4.0
   unified_analytics: ^7.0.0
   vm_service: ^15.0.2
-  vm_service_protos: ^1.0.0
+  vm_service_protos: ^2.0.0
   vm_snapshot_analysis: ^0.7.6
   web: ^1.0.0
   web_socket_channel: ^3.0.0