Update vendored polyglot-sql to 0.8.0

[github-actions]

The upstream Polyglot project has released 0.8.0, newer than the 0.6.2 currently vendored by this package.

• crates.io: https://crates.io/crates/polyglot-sql/0.8.0
• Release notes: https://github.com/tobilg/polyglot/releases/tag/v0.8.0
• Changelog: https://docs.rs/polyglot-sql/0.8.0

How to update

1. tools/update-vendor.sh 0.8.0 — bumps the dependency, refreshes Cargo.lock, re-vendors all crates into src/rust/vendor.tar.xz and regenerates inst/COPYRIGHTS.
2. Set Config/polyglotSQL/upstream: 0.8.0 in DESCRIPTION.
3. Re-check the minimum Rust version printed by the script and keep src/rust/Cargo.toml (rust-version) and DESCRIPTION (SystemRequirements) in sync.
4. Run devtools::test() — a test asserts the linked crate version matches DESCRIPTION.
5. Note the upgrade in NEWS.md.

Opened automatically by the upstream-watch workflow.