[submodule] Update submodule sonic-snmpagent to the latest HEAD automatically (#17274)

#### Why I did it
src/sonic-snmpagent
```
* 3b6a4ad - (HEAD -> master, origin/master, origin/HEAD) Enable faulthandler to provide more fault information (Azure#301) (22 hours ago) [Hua Liu]
```
#### How I did it
#### How to verify it
#### Description for the changelog

Author: mssonicbld