[submodule] Update submodule sonic-snmpagent to the latest HEAD automatically (#16353)

#### Why I did it
src/sonic-snmpagent
```
* af2d5a4 - (HEAD -> master, origin/master, origin/HEAD) Fix FdbUpdater crash when SAI_FDB_ENTRY_ATTR_BRIDGE_PORT_ID attribute missing. (Azure#286) (19 hours ago) [Hua Liu]
```
#### How I did it
#### How to verify it
#### Description for the changelog

Author: mssonicbld