[PM-8814] Add removed EU feature flag to temporarily fix mobile until…

… release rollout (#4177)

cherry picked from commit: 7c80590

src/Core/Constants.cs

@@ -102,6 +102,7 @@ public static class AuthenticationSchemes
 
 public static class FeatureFlagKeys
 {
+    public const string DisplayEuEnvironment = "display-eu-environment";
     public const string BrowserFilelessImport = "browser-fileless-import";
     public const string ReturnErrorOnExistingKeypair = "return-error-on-existing-keypair";
     public const string UseTreeWalkerApiForPageDetailsCollection = "use-tree-walker-api-for-page-details-collection";