Update README instructions from classic token to FGA token

[indcoder]

The current README for the GitHub Analyser Artifact repository still references the use of the classic GitHub token for authentication. As GitHub is deprecating classic tokens in favor of fine-grained access (FGA) tokens, we should update the documentation to reflect this change.

Suggested updates:

• Replace all mentions of "classic token" with "FGA token"
• Include instructions on how to generate and scope an FGA token appropriately for this repo
• Clarify any permission differences or setup steps that may affect users transitioning from classic to FGA tokens

This will help future users avoid confusion and ensure compatibility with GitHub's latest security standards.

[indcoder]

We can follow the same we followed for this PR microsoft/ai-agents-for-beginners#255