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

#17435)

#### Why I did it
src/linkmgrd
```
* e420df4 - (HEAD -> master, origin/master, origin/HEAD) Exclude DbInterface in PR coverage check (#224) (5 hours ago) [Jing Zhang]
```
#### How I did it
#### How to verify it
#### Description for the changelog