Using the GEMINI_API_KEY by default instead of the GOOGLE_API_KEY one

[Giom-V]

Description of the change

Instead of using the GOOGLE_API_KEY by default, we're checking for the GEMINI_API_KEY ones first since its naming is less misleading. The GOOGLE_API_KEY is still kept for consistency.

Motivation

The Google API key can be misleading since it's not really an overall key for all of Google API and only for the Gemini ones.

Type of change

Choose one: Feature request

Checklist

• I have performed a self-review of my code.
• I have added detailed comments to my code where applicable.
• I have verified that my change does not break existing code.
• My PR is based on the latest changes of the main branch (if unsure, please run git pull --rebase upstream main).
• I am familiar with the Google Style Guide for the language I have coded in.
• I have read through the Contributing Guide and signed the Contributor License Agreement.