[submodule] Update submodule sonic-stp to the latest HEAD automatically (sonic-net#21782)

#### Why I did it
src/sonic-stp
```
* 4909e7e - (HEAD -> master, origin/master, origin/HEAD) armhf build on PR validation (sonic-net#48) (8 hours ago) [Divya Kumaran Chandralekha]
```
#### How I did it
#### How to verify it
#### Description for the changelog

Author: mssonicbld