Deprecation of gemini-1.5-flash

[julian-risch]

We're still mentioning the deprecated gemini-1.5-flash model and should removed from two pages:

• https://haystack.deepset.ai/integrations/google-ai#usage
• https://haystack.deepset.ai/integrations/google-vertex-ai#usage

[srini047]

@julian-risch Do you think we have to make use of gemini-2.5-flash in https://github.com/deepset-ai/haystack-core-integrations because I still see we are using that as the default model in google_ai and google_vertex integerations.

[srini047]

@julian-risch Do you think we have to make use of gemini-2.5-flash in https://github.com/deepset-ai/haystack-core-integrations because I still see we are using that as the default model in google_ai and google_vertex integerations.

deepset-ai/haystack-core-integrations#2346
Made a draft PR here incase needed.