refactor(admin): remove isV5Compatible state and changeIsV5Compatible from AdminAppContainer

Now that AppSettingsPageContents reads isV5Compatible via useSWRxAppSettings,
no component reads this value from AdminAppContainer.state. Remove the now-dead
state field (initial + setState in retrieveAppSettingsData) and the unused
changeIsV5Compatible() setter.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

apps/app/src/client/services/AdminAppContainer.js

@@ -23,7 +23,6 @@ export default class AdminAppContainer extends Container {
       isEmailPublishedForNewUser: true,
       isReadOnlyForNewUser: false,
 
-      isV5Compatible: null,
       siteUrl: '',
       siteUrlUseOnlyEnvVars: null,
       envSiteUrl: '',
@@ -68,7 +67,6 @@ export default class AdminAppContainer extends Container {
       globalLang: appSettingsParams.globalLang,
       isEmailPublishedForNewUser: appSettingsParams.isEmailPublishedForNewUser,
       isReadOnlyForNewUser: appSettingsParams.isReadOnlyForNewUser,
-      isV5Compatible: appSettingsParams.isV5Compatible,
       siteUrl: appSettingsParams.siteUrl,
       siteUrlUseOnlyEnvVars: appSettingsParams.siteUrlUseOnlyEnvVars,
       envSiteUrl: appSettingsParams.envSiteUrl,
@@ -127,13 +125,6 @@ export default class AdminAppContainer extends Container {
     this.setState({ isReadOnlyForNewUser });
   }
 
-  /**
-   * Change site url
-   */
-  changeIsV5Compatible(isV5Compatible) {
-    this.setState({ isV5Compatible });
-  }
-
   /**
    * Change site url
    */