Replace deprecated VertexAI gemini-pro with gemini-1.5-pro

[julian-risch]

Is your feature request related to a problem? Please describe.
gemini-1.0-pro support in vertex-ai will end on April 9, 2025
https://cloud.google.com/vertex-ai/generative-ai/docs/deprecations/gemini-1.0-pro
we need to replace it in examples and docs with gemini-1.5-pro

Describe the solution you'd like

• Update docs so that it uses gemini-1.5-pro instead of gemini-1.0-pro https://docs.haystack.deepset.ai/docs/vertexaigeminichatgenerator
• Update test https://github.com/deepset-ai/haystack-core-integrations/blob/main/integrations/google_vertex/tests/test_gemini.py#L203
• Update https://github.com/deepset-ai/haystack-integrations/blob/main/integrations/google-vertex-ai.md

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Additional context
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