Enabling v2 stack as the default for Event Hubs (#43725)

* Enabling v2 stack as the default for Event Hubs

* Update V2StackSupport to enable v2 by default

Author: anuchandy

sdk/eventhubs/azure-messaging-eventhubs/CHANGELOG.md

@@ -4,6 +4,8 @@
 
 ### Features Added
 
+Setting the v2 stack as the default. ([43725](https://github.com/Azure/azure-sdk-for-java/pull/43725))
+
 ### Breaking Changes
 
 - Do not remove `x-opt-partition-key` message annotation when publishing events. 

sdk/eventhubs/azure-messaging-eventhubs/src/main/java/com/azure/messaging/eventhubs/V2StackSupport.java

@@ -35,7 +35,7 @@ final class V2StackSupport {
     private static final ConfigurationProperty<Boolean> V2_STACK_PROPERTY
         = ConfigurationPropertyBuilder.ofBoolean(V2_STACK_KEY)
             .environmentVariableName(V2_STACK_KEY)
-            .defaultValue(false)
+            .defaultValue(true)
             .shared(true)
             .build();
     private final AtomicReference<Boolean> v2StackFlag = new AtomicReference<>();
@@ -62,7 +62,7 @@ final class V2StackSupport {
      * @return true if the clients should use the v2 stack.
      */
     boolean isV2StackEnabled(Configuration configuration) {
-        return isOptedIn(configuration, V2_STACK_PROPERTY, v2StackFlag);
+        return !isOptedOut(configuration, V2_STACK_PROPERTY, v2StackFlag);
     }
 
     /**
@@ -113,33 +113,6 @@ private boolean isOptedOut(Configuration configuration, ConfigurationProperty<Bo
         return choiceFlag.get();
     }
 
-    private boolean isOptedIn(Configuration configuration, ConfigurationProperty<Boolean> configProperty,
-        AtomicReference<Boolean> choiceFlag) {
-        final Boolean flag = choiceFlag.get();
-        if (flag != null) {
-            return flag;
-        }
-
-        final String propName = configProperty.getName();
-        final boolean isOptedIn;
-        if (configuration != null) {
-            isOptedIn = configuration.get(configProperty);
-        } else {
-            assert !CoreUtils.isNullOrEmpty(propName);
-            if (!CoreUtils.isNullOrEmpty(System.getenv(propName))) {
-                isOptedIn = "true".equalsIgnoreCase(System.getenv(propName));
-            } else if (!CoreUtils.isNullOrEmpty(System.getProperty(propName))) {
-                isOptedIn = "true".equalsIgnoreCase(System.getProperty(propName));
-            } else {
-                isOptedIn = false;
-            }
-        }
-        if (choiceFlag.compareAndSet(null, isOptedIn)) {
-            logger.verbose("Selected configuration {}={}", propName, isOptedIn);
-        }
-        return choiceFlag.get();
-    }
-
     ReactorConnectionCache<EventHubReactorAmqpConnection> createConnectionCache(ConnectionOptions connectionOptions,
         Supplier<String> eventHubNameSupplier, MessageSerializer serializer, Meter meter,
         boolean useSessionChannelCache) {