chore(deps): update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yaml

@@ -8,7 +8,7 @@ jobs:
         python-version: [3.8, 3.11]
     steps:
       - name: Check out repository code
-        uses: actions/checkout@v4
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
 
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v5
@@ -37,7 +37,7 @@ jobs:
         node-version: [16]
     steps:
       - name: Check out repository code
-        uses: actions/checkout@v4
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
 
       - name: Set up Node.js ${{ matrix.node-version }}
         uses: actions/setup-node@v4

.github/workflows/integration-test.yaml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
         with:
           path: mitx-grading-library