Update checkout GitHub Action version

[erictleung]

In the workflow files here https://github.com/freeCodeCamp/how-to-contribute-to-open-source/tree/main/.github/workflows, update the actions/checkout@v2 to actions/checkout@v4

For language-check.yml

    steps:
    - name: Checkout repository
-       uses: actions/checkout@v2
+       uses: actions/checkout@v4

For test.yml

    steps:
      - name: Checkout Source Files
-         uses: actions/checkout@v2
+         uses: actions/checkout@v4
        with:
          fetch-depth: 0