chore(README.md): add maintainer google groups for communication channels and remove discussion group (#4093)

Signed-off-by: Gaius <gaius.qi@gmail.com>

Author: gaius-qi

README.md

@@ -62,9 +62,9 @@ you can see the full report [here](docs/security/dragonfly-comprehensive-report-
 Join the conversation and help the community.
 
 - **Slack Channel**: [#dragonfly](https://cloud-native.slack.com/messages/dragonfly/) on [CNCF Slack](https://slack.cncf.io/)
-- **Discussion Group**: <dragonfly-discuss@googlegroups.com>
-- **Developer Group**: <dragonfly-developers@googlegroups.com>
 - **Github Discussions**: [Dragonfly Discussion Forum][discussion]
+- **Developer Group**: <dragonfly-developers@googlegroups.com>
+- **Maintainer Group**: <dragonfly-maintainers@googlegroups.com>
 - **Twitter**: [@dragonfly_oss](https://twitter.com/dragonfly_oss)
 - **DingTalk**: [22880028764](https://qr.dingtalk.com/action/joingroup?code=v1,k1,pkV9IbsSyDusFQdByPSK3HfCG61ZCLeb8b/lpQ3uUqI=&_dt_no_comment=1&origin=11)