Update documentation support links (#1220)

Update support channels in documentation:
- Google Group link removed
- > The Hazelcast Google User Group is read-only. Please post your
questions on the Hazelcast Community Slack)

Author: JackPGreen

Reference_Manual.md

@@ -3843,8 +3843,6 @@ You can use the following channels for your questions and development/usage issu
 
 * This repository by opening an issue.
 * [Hazelcast Community Slack - C++ Client Channel](https://hazelcastcommunity.slack.com/channels/cpp-client)
-* Google Groups: https://groups.google.com/forum/#!forum/hazelcast
-* Stack Overflow: https://stackoverflow.com/questions/tagged/hazelcast
 
 # 10. Contributing
 

hazelcast/include/hazelcast/client/hazelcast_client.h

@@ -71,7 +71,7 @@ namespace client {
  * Our C++ client is completely open source and the source code is freely
  * available at https://github.com/hazelcast/hazelcast-cpp-client . Please feel
  * free to contribute. You can join our community at
- * https://groups.google.com/forum/#!forum/hazelcast where you can find answers
+ * https://hazelcastcommunity.slack.com/channels/cpp-client where you can find answers
  * to your questions.
  */
 class HAZELCAST_API hazelcast_client