prepare for 8.0.0 platform interface release (#559)

Author: MaikuB

flutter_appauth_platform_interface/CHANGELOG.md

@@ -1,4 +1,4 @@
-## [8.0.0-dev.1]
+## [8.0.0]
 
 * **Breaking change** Replaced the `preferEphemeralSession` property in the `AuthorizationRequest`, `AuthorizationTokenRequest` and  `EndSessionRequest` classes with `externalUserAgent`. Thanks to the PR from [john-slow](https://github.com/john-slow). `externalUserAgent` is presented by the newly `ExternalUserAgent` enum that has the following values
     * `asWebAuthenticationSession`: uses the [ASWebAuthenticationSession](https://developer.apple.com/documentation/authenticationservices/aswebauthenticationsession) APIs where possible. This is the default value and was the default behaviour behaviour that aligns with what the AppAuth iOS SDK would do in choosing the best available user-agent

flutter_appauth_platform_interface/pubspec.yaml

@@ -1,6 +1,6 @@
 name: flutter_appauth_platform_interface
 description: A common platform interface for the flutter_appauth plugin.
-version: 8.0.0-dev.1
+version: 8.0.0
 homepage: https://github.com/MaikuB/flutter_appauth/tree/master/flutter_appauth_platform_interface
 
 environment: