Update AG2 logo to be a Git LFS pointer

[marklysze]

Why are these changes needed?

The current AG2 logo isn't stored as an LFS pointer.

Related issue number

N/A

Checks

• I've included any doc changes needed for https://ag2ai.github.io/ag2/. See https://ag2ai.github.io/ag2/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.

[qingyun-wu]

LGTM! Thanks Mark!