Migrating away from google-generativeai

Hi team, Mark here from the Gemini DevX team. I'm hunting down significant usage of our deprecated [`google-generativeai`](https://pypi.org/project/google-generativeai) SDK and noticed your `geemap.ai` library using it. This SDK is deprecated, so it emits warnings on import, won't receive bugfixes and does not support our latest model features so it's important we switch to the newer `google-genai` SDK. I've made a first pass (not ready for merging or anything) here: https://github.com/gee-community/geemap/compare/master...markmcd:geemap:genai-migro but it will need a bit more work, so I wanted to check in and see if you're open to this update first. WDYT?