[google_maps_flutter_web] update min flutter sdk version to 1.20.0 (#3662)

packages/google_maps_flutter/google_maps_flutter_web/CHANGELOG.md

diff --git a/packages/google_maps_flutter/google_maps_flutter_web/CHANGELOG.md b/packages/google_maps_flutter/google_maps_flutter_web/CHANGELOG.md
index f5e289c..4277d7b 100644
--- a/packages/google_maps_flutter/google_maps_flutter_web/CHANGELOG.md
+++ b/packages/google_maps_flutter/google_maps_flutter_web/CHANGELOG.md
@@ -1,6 +1,6 @@
 ## 0.1.2
 
-* Add support for Custom Tiles.
+* Update min Flutter SDK to 1.20.0.
 
 ## 0.1.1
 

packages/google_maps_flutter/google_maps_flutter_web/pubspec.yaml

diff --git a/packages/google_maps_flutter/google_maps_flutter_web/pubspec.yaml b/packages/google_maps_flutter/google_maps_flutter_web/pubspec.yaml
index 83ac0fb..595ca20 100644
--- a/packages/google_maps_flutter/google_maps_flutter_web/pubspec.yaml
+++ b/packages/google_maps_flutter/google_maps_flutter_web/pubspec.yaml
@@ -33,4 +33,4 @@
 
 environment:
   sdk: ">=2.3.0 <3.0.0"
-  flutter: ">=1.10.0"
+  flutter: ">=1.20.0"