Bump CI node version to 20 (#2217)

To get rid of deprecation warnings, eg
https://github.com/cursorless-dev/cursorless/actions/runs/7742075291.
See
https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/
for more info

## Checklist

- [-] I have added
[tests](https://www.cursorless.org/docs/contributing/test-case-recorder/)
- [-] I have updated the
[docs](https://github.com/cursorless-dev/cursorless/tree/main/docs) and
[cheatsheet](https://github.com/cursorless-dev/cursorless/tree/main/cursorless-talon/src/cheatsheet)
- [-] I have not broken the cheatsheet

Author: pokey

.github/workflows/black.yml

@@ -6,5 +6,5 @@ jobs:
   lint:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: psf/black@stable