[Tasks] Rename generate property in task specs

[hanouticelina]

Fixes #923

This PR updates the task specs to rename the generate property to generation_parameters. This change aligns with the discussion in the issue.

Key changes:

• Renamed generate to generation_parameters in the specs for automatic-speech-recognition, image-to-text, text-to-audio and text-to-speech tasks.

[Wauplin]

Looks good, thanks!

For other reviewers, Inference API / Inference Endpoints won't support generation_parameters yet but since they already don't support generate, that's not a breaking change 😄