Fix SARIF schema for VS Code; upgrade to Frama-C 23.1

This release fixes the following issues:

• Updates the SARIF $schema URL, to improve compatibility with VS Code;
• Uses a newer, Alpine-based Frama-C Docker image to run the analyses. This image is based on Frama-C 23.1+dev (development version after the release of Frama-C 23.1 (Vanadium)).