README.md: Update GitHub Actions (#259)

* https://github.com/actions/checkout/releases
* https://github.com/actions/setup-node/releases

Encourage readers to use up-to-date GitHub Actions.

Author: cclauss

README.md

@@ -58,10 +58,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
 
     - name: Install Node.js
-      uses: actions/setup-node@v4
+      uses: actions/setup-node@v6
       with:
         node-version: lts/*
 
@@ -90,9 +90,9 @@ jobs:
           - 1
           - 2
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - name: Install Node.js
-      uses: actions/setup-node@v4
+      uses: actions/setup-node@v6
       with:
         node-version: lts/*