Use google-genai package instead of old google-generativeai package

[kumaranvpl]

Why are these changes needed?

Related issue number

Closes #599

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/docs/contributor-guide/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.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 4.34783% with 22 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
autogen/oai/gemini.py	0.00%	21 Missing ⚠️
autogen/oai/client.py	50.00%	1 Missing ⚠️

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Flag	Coverage Δ
unittests	13.57% <4.34%> (-29.90%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
autogen/oai/client.py	23.26% <50.00%> (-28.53%)	⬇️
autogen/oai/gemini.py	0.00% <0.00%> (-13.43%)	⬇️

... and 140 files with indirect coverage changes