Update checkout action's version

Author: mw-hrastega

README.md

@@ -21,7 +21,7 @@ jobs:
     runs-on: self-hosted
     steps:
       - name: Check out repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Run build
         uses: matlab-actions/run-build@v3
 ```
@@ -38,7 +38,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Check out repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Set up MATLAB
         uses: matlab-actions/setup-matlab@v3
       - name: Run build
@@ -68,7 +68,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Check out repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Set up MATLAB
         uses: matlab-actions/setup-matlab@v3
       - name: Run build