Update vm_service to '>=8.1.0 <9.0.0' (#357)

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index 8563b3b..52fef87 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -13,7 +13,7 @@
   path: ^1.8.0
   source_maps: ^0.10.10
   stack_trace: ^1.10.0
-  vm_service: ">=7.3.0 <8.0.0"
+  vm_service: ">=8.1.0 <9.0.0"
 dev_dependencies:
   lints: ^1.0.0
   test: ^1.16.0