Clean-up and unify CI workflows

[krant]

Opinionated maintenance pass over CI:

• Unify "Testing" and "Hygiene" workflows. It gives single environment per OS and declutters Actions UI.
• Upgrade to ubuntu-latest runner.
• Remove actions/cache altogether. It was not working anyway (Cache not found on each CI run) and it makes little sense for projects without Cargo.lock.
• Fix cache by using properly set-up Swatinem/rust-cache@v2
• Remove dtolnay/rust-toolchain - Github runners are already have latest stable Rust.
• Remove pnpm setup - I have not found why it's needed, tests are passing without it.
• Simpler wording for less UI noise (Testing Suite -> Testing, unit-test -> test, etc.).
• CARGO_REGISTRIES_CRATES_IO_PROTOCOL: sparse is enabled by default already.

We will need more features from CI in the near future - let's build them on top of minimal and clean YAML.