Adapt BioChatter-light to Gemini

[fcarli]

Given that Gemini 2.0 flash (also 2.5) allow for a free tier usage, migrate the BioChatter-light interface to use this new model

[fcarli]

Code almost ready for a PR in https://github.com/biocypher/biochatter-light/tree/dev-gemini

• Set gemini-2.0-flash as the default model
• Removed the token tracking from the interface

[fcarli]

Opened a PR (biocypher/biochatter-light#63)