Update materials to access ChatMessage text (instead of content)

[anakin87]

part of #8583

Since content will be removed in future in favor of text (and other types of content),
we are already transitioning to text to smooth the transition process.

Materials to update

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1. tutorials
2. cookbook (in review...)
3. integration pages (in review...)
4. blogposts (in review...)

[anakin87]

About tutorials: the only tutorial affected is https://haystack.deepset.ai/tutorials/40_building_chat_application_with_function_calling.
I leave it unchanged for now, since it will require a complete refactoring once we introduce the new ChatMessage and Tool in 2.9.0.

Docs: I created #8623 to keep track of all the actions needed on the documentation side. No need to do this in the current issue.

[anakin87]

done in deepset-ai/haystack-cookbook#156, deepset-ai/haystack-integrations#290, and deepset-ai/haystack-home#392