fix: allow release-plz to use crates.io registry

[f0rr0]

Summary

• remove the manifest alternate-registry publish list so release-plz uses Cargo's default crates.io registry
• document why package.publish = ["crates-io"] should not be reintroduced

Verification

• taplo fmt --check Cargo.toml release-plz.toml
• cargo metadata --no-deps --format-version 1 reports publish: null for pglite-oxide
• cargo package --locked --allow-dirty
• cargo publish --locked --dry-run --allow-dirty
• cargo check --all-targets --locked

The failed publish-dry-run workflow passed all package checks and failed only in release-plz with The registry 'crates-io' could not be found. After this merges, rerun Release from main with publish-dry-run, then publish if it passes.