[submodule] Update submodule sonic-stp to the latest HEAD automatically (sonic-net#21970)

#### Why I did it
src/sonic-stp
```
* f3dfb2c - (HEAD -> master, origin/master, origin/HEAD) Mstp Feature - Adding library and DB management APIs (sonic-net#50) (14 hours ago) [Divya Kumaran Chandralekha]
* 38ac0f1 - MSTP header files (sonic-net#49) (23 hours ago) [Divya Kumaran Chandralekha]
```
#### How I did it
#### How to verify it
#### Description for the changelog

Author: mssonicbld