chore: require at least Node.js version v20 and pnpm v9.

The project is set up to target Node v20, as indicated by the Node types
we use. Thus, this commit:

- Sets the minimum Node.js version to v20.
- Sets the minimum pnpm version to v9.
- Requires Node v20 instead of v22 in the CI.

Change-Id: Icf9cb245fc4632baff8a2db9f2e85049cb4af010
diff --git a/playbooks/nightly-build/pre.yaml b/playbooks/nightly-build/pre.yaml
index 8af7bb2..4557d37 100644
--- a/playbooks/nightly-build/pre.yaml
+++ b/playbooks/nightly-build/pre.yaml
@@ -5,6 +5,6 @@
     - role: ensure-genmanifest
     - role: ensure-nodejs-fork
       vars:
-        node_version: 22
+        node_version: 20
     - role: ensure-cws-upload
       version: "2.1.0"