Changed API path to v2 instead of v2.0

We discussed whether the minor-version is relevant for the API base-path and came to the conclusion, that it's not really needed as breaking API changes need a new major version anyway.
This commit also removes the trailing slash from the URL which is returned by `getBackendUrl`. This is needed as we composed the API routes in our implementation always with a starting slash thus leading to double slashes.

Example of old behaviour:
getBackendUrl() + '/config'
=> example.com/api/v2.0//config

Example of new behaviuor:
getBackendUrl() + '/config'
=> example.com/api/v2/config

src/redux/frontend-config/methods.ts

@@ -7,7 +7,7 @@ export const setFrontendConfig = (state: FrontendConfig): void => {
     type: FrontendConfigActionType.SET_FRONTEND_CONFIG,
     state: {
       ...state,
-      backendUrl: state.backendUrl + '/api/v2.0/'
+      backendUrl: state.backendUrl + '/api/v2'
     }
   }
   store.dispatch(action)