update githib actions/checkout from v3 to v7

To resolve the following warning on master build:
Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

v5 updates node but instead updating to the latest as reading the release notes (https://github.com/actions/checkout) v7 has a security fix for avoiding forks exfiltrating build secrets.

Author: AaronRobson

.github/workflows/jekyll.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v7
     - name: Build the site in the jekyll/builder container
       run: |
         docker run \