[submodule] Update submodule linkmgrd to the latest HEAD automatically (

#18585)

#### Why I did it
src/linkmgrd
```
* 7706c2c - (HEAD -> 202305, origin/202305) [active-standby] reset mux probing backoff factor in link flaps (#245) (2 days ago) [Jing Zhang]
```
#### How I did it
#### How to verify it
#### Description for the changelog