[submodule] Update submodule dhcpmon to the latest HEAD automatically (sonic-net#21780)

#### Why I did it
src/dhcpmon
```
* f6dfbe9 - (HEAD -> master, origin/master, origin/HEAD) Fix counting bootp packets by mistake (sonic-net#31) (19 hours ago) [Yaqiang Zhu]
```
#### How I did it
#### How to verify it
#### Description for the changelog

Author: mssonicbld