[submodule] Update submodule sonic-snmpagent to the latest HEAD automatically (#22521)

#### Why I did it
src/sonic-snmpagent
```
* 13b975b - (HEAD -> master, origin/master, origin/HEAD) [rfc1213] Handle error seen when interface counter is not available in COUNTERS_DB (#351) (4 hours ago) [SuvarnaMeenakshi]
```
#### How I did it
#### How to verify it
#### Description for the changelog

Author: mssonicbld