[path_provider] Bump platform interface package to stable NNBD (#3568)

Author: stuartmorgan-g

packages/path_provider/path_provider_platform_interface/CHANGELOG.md

@@ -1,4 +1,4 @@
-## 2.0.0-nullsafety
+## 2.0.0
 
 * Migrate to null safety.
 

packages/path_provider/path_provider_platform_interface/pubspec.yaml

@@ -3,20 +3,20 @@ description: A common platform interface for the path_provider plugin.
 homepage: https://github.com/flutter/plugins/tree/master/packages/path_provider/path_provider_platform_interface
 # NOTE: We strongly prefer non-breaking changes, even at the expense of a
 # less-clean API. See https://flutter.dev/go/platform-interface-breaking-changes
-version: 2.0.0-nullsafety
+version: 2.0.0
 
 dependencies:
   flutter:
     sdk: flutter
-  meta: ^1.3.0-nullsafety.3
-  platform: ^3.0.0-nullsafety.4
-  plugin_platform_interface: ^1.1.0-nullsafety
+  meta: ^1.3.0
+  platform: ^3.0.0
+  plugin_platform_interface: ">=1.0.0 <3.0.0"
 
 dev_dependencies:
   flutter_test:
     sdk: flutter
-  pedantic: ^1.10.0-nullsafety.1
+  pedantic: ^1.10.0
 
 environment:
-  sdk: ">=2.12.0-0 <3.0.0"
+  sdk: ">=2.12.0-259.9.beta <3.0.0"
   flutter: ">=1.10.0"