[submodule] Update submodule sonic-stp to the latest HEAD automatically (#24633)

#### Why I did it
src/sonic-stp
```
* 49c02e5 - (HEAD -> master, origin/master, origin/HEAD) Migrate agent pool from ubuntu-20.04 to ubuntu-latest (#74) (25 minutes ago) [yijingyan2]
* 6dc2791 - Enable proto_mode handling code (#75) (2 days ago) [Divya Kumaran Chandralekha]
```
#### How I did it
#### How to verify it
#### Description for the changelog

Author: mssonicbld