[submodule] Update submodule dhcpmon to the latest HEAD automatically (#25508)

#### Why I did it
src/dhcpmon
```
* d78974f - (HEAD -> master, origin/master, origin/HEAD) [dhcp_mon] Add missing #include <algorithm> for std::replace (#68) (31 hours ago) [rustiqly]
* d4934fa - Add .github/copilot-instructions.md for AI-assisted development (#67) (31 hours ago) [rustiqly]
```
#### How I did it
#### How to verify it
#### Description for the changelog

Author: mssonicbld