Replace `serde_yml` unmaintained crate

[jpraynaud]

Why

The crate serde_yml is unmaintained and unsound as detailed in the See security advisory and the GitHub repository. We need to use another stable crate to do the yaml parsing in Mihtril build scripts and tests for OpenAPI specs.

Note

We have already switched from serde_yaml to serde_yml for the same reason as stated in the issue #2639

What

Replace serde_yml with a safe and maintained crate.

How

• Identify a new crate that is maintained:
  • saphyr
  • rust-yaml (less mature, less contributors than saphyr)
• Replace serde_yml with the new crate:
  • internal/mithril-build-script
  • internal/mithril-api-spec