chore: upstream dfinity/cdk-rs moved to 0.20.2

[pr-automation-bot-public]

dfinity/cdk-rs has moved past the ref the docs are verified against.

Pinned in .sources/upstream.json	0.20.1
Latest crates.io release of ic-cdk	0.20.2
Registry	https://crates.io/crates/ic-cdk
Published reference	https://docs.rs/ic-cdk/latest/ic_cdk/

What to re-check

ic-cdk APIs in Rust code blocks. The repo stopped tagging releases (its newest bare-semver tag is two minors behind the published crate), so the crate version on crates.io is the release identity. Read the sections newer than the pin in https://github.com/dfinity/cdk-rs/blob/master/ic-cdk/CHANGELOG.md, then grep docs/ for the symbols they name. ic-cdk-timers and ic-cdk-executor version separately; check whether they moved too.

How to close this

1. Review the changes above against the pages named under "What to re-check".
2. Update those pages if anything they state has changed.
3. Set pinned to 0.20.2 in .sources/upstream.json in the same PR.

If nothing in the docs is affected, bump the pin on its own and say so in the PR.

Procedure: .agents/upstream-tracking.md

[marc0olo]

Done in #370. No doc impact: 0.20.2 only makes update, query, init and post_upgrade entry points treat an empty input vector as the Candid encoding of (), which no example depends on. Pin moved to 0.20.2.