Updating to v4 for checkout and coverage actions

GitHub warning prompted this change:
  Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20:
  actions/checkout@v3, codecov/codecov-action@v3.
  Node.js 16 actions are deprecated. For more information see:
  https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

.github/workflows/unit-test.yml

@@ -12,7 +12,7 @@ jobs:
         python-version: ['3.9', '3.10', '3.11']
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Set up OnAIR test environment
       uses: conda-incubator/setup-miniconda@v3
       with:
@@ -32,4 +32,4 @@ jobs:
       shell: bash -l {0}
       run: coverage report --skip-empty
     - name: Upload coverage reports to Codecov
-      uses: codecov/codecov-action@v3
+      uses: codecov/codecov-action@v4