Update GitHub CI to use v3 of actions/checkout

Author: rdzman

.github/workflows/continuous-integration.yml

@@ -23,7 +23,7 @@ jobs:
 
     steps:
     - name: Check out repository
-      uses: actions/checkout@v2
+      uses: actions/checkout@v3
 
     - name: Install Octave (Linux)
       if: matrix.platform == 'octave' && startsWith(matrix.os, 'ubuntu')