ci: replace deprecated Node 20 actions in rust.yml

[divybot]

Summary

GitHub Actions will force any step still on Node 20 to Node 24 starting 2026-06-02. The three actions pinned in .github/workflows/rust.yml are all Node 20 runtimes today; one (actions-rs/toolchain) has been archived since 2022 and is the most likely to misbehave under the forced Node 24 runtime.

This bumps them to maintained equivalents:

• actions/checkout@v3 → @v4
• actions-rs/toolchain@v1 → dtolnay/rust-toolchain@master (the common maintained replacement; preserves the existing toolchain: 1.92.0 pin and the rustfmt component; override: true is dropped because dtolnay/rust-toolchain already sets the toolchain as the rustup default)
• denoland/setup-deno@v1 → @v2

No matrix/feature changes — Build, Test, Check formatting, and Autobahn|Testsuite continue to run across ubuntu-latest / macos-latest / windows-latest as before.

Closes denoland/orchid#238

Test plan

• CI green on ubuntu-latest, macos-latest, windows-latest
• Autobahn|Testsuite step still runs on ubuntu-latest

[divybot]

CI note: across the three runs on this PR, every platform has passed at least once. The remaining red checks are all transient codeload.github.com download failures during GitHub Actions' "Set up job" phase (before any of the workflow's own steps run), with errors of the form:

##[error]An action could not be found at the URI 'https://codeload.github.com/denoland/setup-deno/...'
Failed to download archive '...' after 1 attempts.

The action SHAs resolve fine and the archives are valid (verified out-of-band); this is a GitHub-side infra flake, not a workflow defect.

Tally of Build / Test / Check formatting / Autobahn|Testsuite (the actual workflow steps) across the three runs:

OS	Pass	Fail-in-setup-phase
ubuntu-latest	2	1
macos-latest	2	1
windows-latest	1	2

Every OS has had a green end-to-end run with the bumped actions. Happy to push another retrigger if reviewers want a clean three-green snapshot — would prefer not to keep amending purely to dodge GH's codeload weather.