Use string as scope

Using a list of scopes was not spec compliant: thephpleague/oauth2-server#1469

With an update of our oauth lib this is no longer supported, leading to fails in the ATS: shopware/shopware#5986

src/services/AdminApiContext.ts

@@ -89,7 +89,7 @@ export class AdminApiContext {
                 grant_type: 'client_credentials',
                 client_id: options.client_id,
                 client_secret: options.client_secret,
-                scope: ['write'],
+                scope: 'write',
             },
         });
 
@@ -109,7 +109,7 @@ export class AdminApiContext {
                 grant_type: 'password',
                 username: options.admin_username,
                 password: options.admin_password,
-                scope: ['write'],
+                scope: 'write',
             },
         });
 
@@ -150,4 +150,4 @@ export class AdminApiContext {
     async head<PAYLOAD>(url: string, options?: RequestOptions<PAYLOAD>): Promise<APIResponse> {
         return this.context.head(url, options);
     }
-}
+}