[Refactor] fix quote style in template (#127762)

*Just replaced double quote to single.*

*this looks weird to me.*
![screenshot](https://github.com/flutter/flutter/assets/50433979/0bed4730-7fce-411e-b12f-24d478be13c3)

packages/flutter_tools/templates/plugin_shared/pubspec.yaml.tmpl

@@ -5,7 +5,7 @@ homepage:
 
 environment:
   sdk: {{dartSdkVersionBounds}}
-  flutter: ">=3.3.0"
+  flutter: '>=3.3.0'
 
 dependencies:
   flutter: