release: prepare gzp 2.0.4

[sstadick]

Summary

• bump the crate to 2.0.4 and document the release
• consolidate the open Dependabot updates for bytes, thiserror, libdeflater, proptest, and tempfile
• update the pinned actions/checkout action to v7.0.1
• move the lockfile from the yanked spin 0.9.8 release to 0.9.9

This supersedes #72, #73, #74, #75, #76, and #77.

Validation

• cargo fmt --all -- --check
• cargo check --locked --all-features
• cargo clippy --locked --all-features -- -D warnings
• cargo test --locked --all-features
• cargo test --locked --all-features -- --ignored
• cargo test --release --locked --no-default-features --features deflate_rust
• cargo package --locked --allow-dirty
• Criterion comparison against v2.0.3 across Gzip and Snappy with 0–32 workers: no statistically significant regressions detected

Release

After merge, publish gzp 2.0.4 to crates.io.