chore: update runtime to node20

[erikburt]

See #38 for more context/motivation.

Changes:

1. Updated action runtime to node20
2. npm ci
3. npm run build - rebuilding dist using node 20.8.1
4. Updated docs

Tested by running locally with node20: RUNNER_TEMP=/tmp INPUT_VERSION=nightly node dist/index.js

Output

❯ RUNNER_TEMP=/tmp INPUT_VERSION=nightly node dist/index.js
Downloading Foundry 'nightly' from: https://github.com/foundry-rs/foundry/releases/download/nightly/foundry_nightly_darwin_arm64.tar.gz
::debug::Downloading https://github.com/foundry-rs/foundry/releases/download/nightly/foundry_nightly_darwin_arm64.tar.gz
::debug::Destination /tmp/22dc5b3b-bd15-411c-8451-0c668ee46518
::debug::download complete
::debug::Extracting /tmp/22dc5b3b-bd15-411c-8451-0c668ee46518
::debug::Checking tar --version
::debug::bsdtar 3.5.3 - libarchive 3.5.3 zlib/1.2.11 liblzma/5.0.5 bz2lib/1.0.8
[command]/usr/bin/tar xz -C /tmp/78749109-7d11-42a1-ba5e-69f0877f36dd -f /tmp/22dc5b3b-bd15-411c-8451-0c668ee46518
::add-path::/tmp/78749109-7d11-42a1-ba5e-69f0877f36dd

---

Closes #38.

[Evalir]

This modifies both the action.yml file and the readme file, so this should be correct. looping @onbjerg for a last check—we'd need to release a new version

[onbjerg]

lgtm provided ci passes

[erikburt]

Any updates :)