sync: reconcile rustnzb monorepo drift

[thedancingdeveloper]

Summary

• reconcile the standalone crate with rustnzb/crates/nzb-nntp through the monorepo release/v1.3.9 tip (60c5bb4)
• carry forward the newer NNTP auth/error handling, default pipelining, startup-cap removal, body-buffer pooling, mock-server controls, and regression coverage
• remove the obsolete global connection gate while preserving the standalone repo’s ring-only TLS dependency configuration
• bump the publishable crate version from 0.2.20 to 0.2.23 (matching and superseding the latest immutable crates.io release)

Validation

• cargo fmt --all -- --check
• cargo check --workspace --all-features --locked
• cargo test --workspace --all-features --locked (187 passed, 6 live-provider tests ignored)
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo package --allow-dirty

Migration / dependency automation

• canonical target verified as TheDancingDeveloper-org/nzb-nntp; the legacy organization URL redirects here
• weekly Cargo and GitHub Actions Dependabot configuration is present
• vulnerability alerts and automated security fixes are enabled