Lower minimum dart version to 2.12

Author: agilob

modules/openapi-generator/src/main/resources/dart2/pubspec.mustache

@@ -7,7 +7,7 @@ version: '{{{pubVersion}}}'
 description: '{{{pubDescription}}}'
 homepage: '{{{pubHomepage}}}'
 environment:
-  sdk: '>=2.14.0 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 dependencies:
   http: '>=0.13.0 <0.14.0'
   intl: '^0.17.0'

samples/openapi3/client/petstore/dart2/petstore_client_lib/pubspec.yaml

@@ -7,7 +7,7 @@ version: '1.0.0'
 description: 'OpenAPI API client'
 homepage: 'homepage'
 environment:
-  sdk: '>=2.14.0 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 dependencies:
   http: '>=0.13.0 <0.14.0'
   intl: '^0.17.0'

samples/openapi3/client/petstore/dart2/petstore_client_lib_fake/pubspec.yaml

@@ -7,7 +7,7 @@ version: '1.0.0'
 description: 'OpenAPI API client'
 homepage: 'homepage'
 environment:
-  sdk: '>=2.14.0 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 dependencies:
   http: '>=0.13.0 <0.14.0'
   intl: '^0.17.0'