Remove bincode dev-dependency from serde_bytes docs/examples

[Uhm-why]

This PR removes bincode from serde_bytes dev-dependencies and updates docs/examples to avoid depending on bincode.

Why

bincode is currently only used as a dev/doc dependency in this repository, but it is flagged by RustSec as unmaintained:

• RustSec: https://rustsec.org/advisories/RUSTSEC-2025-0141.html

Downstream distributions are tracking and remediating this across Rust packages, including Fedora:

• Fedora Forge tracker: https://forge.fedoraproject.org/rust/backlog/issues/6

This change is intended to reduce downstream security-policy/audit friction without changing crate API or runtime behavior.

What changed

• Removed bincode from [dev-dependencies] in Cargo.toml.
• Updated serde_bytes docs/examples that previously used bincode::serde::* to serializer-agnostic examples (no specific format dependency).
• Removed CI outdated ignore for bincode (--ignore bincode) since it is no longer a dependency.

Scope / impact

• No changes to public API.
• No changes to runtime behavior of serde_bytes.
• Change is limited to development/docs/CI dependency surface.

Related discussions/issues

• serde-rs/bytes issue Clarification on Status of Bincode #60
• RustSec advisory: https://rustsec.org/advisories/RUSTSEC-2025-0141.html
• Fedora Forge backlog issue: https://forge.fedoraproject.org/rust/backlog/issues/6