chore: bump flutter minimum version for web packages (#9748)

Author: russellwheatley

packages/cloud_firestore/cloud_firestore_web/pubspec.yaml

@@ -7,7 +7,7 @@ version: 3.0.0
 
 environment:
   sdk: ">=2.16.0 <3.0.0"
-  flutter: ">=1.12.13+hotfix.4"
+  flutter: ">=1.20.0"
 
 dependencies:
   _flutterfire_internals: ^1.0.3

packages/cloud_functions/cloud_functions_web/pubspec.yaml

@@ -7,7 +7,7 @@ version: 4.3.8
 
 environment:
   sdk: ">=2.16.0 <3.0.0"
-  flutter: ">=1.12.13+hotfix.4"
+  flutter: ">=1.20.0"
 
 dependencies:
   cloud_functions_platform_interface: ^5.1.19

packages/firebase_core/firebase_core_web/pubspec.yaml

@@ -6,7 +6,7 @@ version: 2.0.0
 
 environment:
   sdk: ">=2.16.0 <3.0.0"
-  flutter: ">=1.12.13+hotfix.5"
+  flutter: ">=1.20.0"
 
 dependencies:
   firebase_core_platform_interface: ^4.5.1

packages/firebase_messaging/firebase_messaging_web/pubspec.yaml

@@ -6,7 +6,7 @@ version: 3.2.1
 
 environment:
   sdk: '>=2.16.0 <3.0.0'
-  flutter: ">=1.12.13+hotfix.4"
+  flutter: ">=1.20.0"
 
 dependencies:
   _flutterfire_internals: ^1.0.3