move the package pyautogen to autogen

[Hk669]

Why are these changes needed?

moving the package from pyautogen to autogen for all the existing notebooks in the cookbook.

Related issue number

Checks

• I've included any doc changes needed for https://autogen-ai.github.io/autogen/. See https://autogen-ai.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[sonichi]

The change for notebooks can be made in the autogen repo for now.

[marklysze]

Looks good :)