Rename sandbox to staging environment

To match the upstream name

src/main/java/org/asamk/signal/App.java

@@ -155,7 +155,7 @@ public class App {
         final var serviceEnvironmentCli = ns.<ServiceEnvironmentCli>get("service-environment");
         final var serviceEnvironment = serviceEnvironmentCli == ServiceEnvironmentCli.LIVE
                 ? ServiceEnvironment.LIVE
-                : ServiceEnvironment.SANDBOX;
+                : ServiceEnvironment.STAGING;
 
         final var trustNewIdentityCli = ns.<TrustNewIdentityCli>get("trust-new-identities");
         final var trustNewIdentity = trustNewIdentityCli == TrustNewIdentityCli.ON_FIRST_USE

src/main/java/org/asamk/signal/ServiceEnvironmentCli.java

@@ -7,7 +7,13 @@ public enum ServiceEnvironmentCli {
             return "live";
         }
     },
-    SANDBOX {
+    STAGING {
+        @Override
+        public String toString() {
+            return "staging";
+        }
+    },
+    @Deprecated SANDBOX {
         @Override
         public String toString() {
             return "sandbox";