[lopatnov]

Refs #779 (replace the deprecated serde_yaml; the maintainers' answer there was that they would). Fixes #1058 (RUSTSEC-2024-0449).

Why

serde_yaml is archived, and it pulls in unsafe-libyaml, which the advisory RUSTSEC-2024-0449 reports as unmaintained. It reaches every downstream project through pingora-core, so each of them has to ignore the advisory (#1058). serde-saphyr is safe Rust, maintained, and the replacement the advisory suggests.

What

• pingora-core: ServerConf::from_yaml and ServerConf::to_yaml use serde_saphyr::from_str / to_string. The API of pingora-core does not change.
• pingora-proxy: the modify_response example (and the dev-dependency) uses serde_saphyr::to_string.
• The one test that inspected a serde_yaml::Value (test_working_directory_deserializes_from_yaml_string) now checks the written yaml and that it reads back as the same configuration.
• A second commit changes the CI toolchain matrix (see below).

serde_yaml and unsafe-libyaml are gone from the dependency tree of pingora-core and pingora-proxy (cargo tree -i unsafe-libyaml finds nothing).

The MSRV (please read)

serde-saphyr uses let chains and declares rust-version = "1.89" (its releases from 1.2.0 on; 1.1.0 declares nothing but already fails on 1.85 with error[E0658]: let expressions in this position are unstable). The job that checks the oldest toolchain, 1.85.0, therefore cannot build pingora-core with it. This was the question raised in #779 ("it requires rust 1.89").

I kept that change in its own commit, so it is easy to drop or rework: the oldest toolchain in .github/workflows/build.yml becomes 1.89.0 (same exclusions as before), and pingora-cache, which declares a rust-version and depends on pingora-core, declares 1.89. I did not touch clippy.toml (msrv = "1.85"), because changing it can enable new lints, which is a separate decision. If you prefer to keep 1.85, the alternative is a maintained fork of the same libyaml port (for example serde_norway), which keeps the C-translated parser and may build on 1.85; I did not try it. Tell me which way you prefer and I will rework the PR.

Checked

• cargo test -p pingora-core --lib server::configuration: 14 passed, on stable (1.99.1, Linux).
• cargo check -p pingora-core -p pingora-proxy -p pingora-cache -p pingora-load-balancing --all-targets and cargo check -p pingora-proxy --example modify_response on 1.89.0 and on stable.
• cargo fmt --all -- --check is clean.
• Not run here: cargo check --workspace (the BoringSSL crates need libclang, which my machine lacks), the TLS-backend tests, the 1.89.0 test leg of CI.

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