CI: Bump actions/checkout from 2 to 4

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Commits](actions/checkout@v2...v4)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/run_tests.yml

@@ -11,7 +11,7 @@ jobs:
       matrix:
         python-version: ["3.10"]  # save action minutes; not relevant: for open source unlimited minutes
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Set up Python ${{matrix.python-version}}
         uses: actions/setup-python@v2
         with:
@@ -36,7 +36,7 @@ jobs:
   #     matrix:
   #       python-version: ["3.8"]
   #   steps:
-  #     - uses: actions/checkout@v2
+  #     - uses: actions/checkout@v4
   #     - name: Check disk space
   #       run: df -h
   #     - name: Login to Docker Hub

.github/workflows/run_tests_os.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
 
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v2