Replace once_cell with standard library primitives

[ting-hong-shieh]

Closes #721.

This is a current-main replacement for #732 and #735, which now conflict with the base branch.

Summary

• Replace Pingora's direct uses of once_cell::sync::Lazy and OnceCell with std::sync::LazyLock and OnceLock.
• Preserve concurrent runtime initialization by using OnceLock::set followed by get, and add a test that initializes a no-steal runtime from four threads.
• Update examples and user guides to use the standard-library types, then remove the direct once_cell declarations from the workspace crates.

once_cell can still appear transitively through third-party crates; this change removes Pingora's direct dependency on it.

Testing

• cargo fmt --all -- --check
• cargo +1.85.0 check --workspace --exclude pingora-foundations
• cargo check --workspace
• cargo test -p pingora-runtime test_no_steal_runtime_concurrent_initialization -- --nocapture
• cargo test --doc
• cargo clippy --all-targets --all -- --allow=unknown-lints --deny=warnings
• cargo machete

The full library, binary, and integration test command passed all targets that do not require OpenResty. The local host does not have OpenResty, so pingora-proxy's test_basic and test_upstream targets could not start their mock origin; the repository CI installs OpenResty before running them.