workflows: update actions/checkout to v4 (#63)

No breaking changes.  v3 uses an old version of Node that will start
throwing deprecation warnings soon:

https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/

Author: bgilbert

.github/workflows/codespell.yml

@@ -11,6 +11,6 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Codespell
         uses: codespell-project/actions-codespell@v2

.github/workflows/run_unit_tests.yml

@@ -21,7 +21,7 @@ jobs:
           - os: windows-latest
             cc: clang
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
 
     - name: Install build and test dependencies (Linux)
       if: startsWith(matrix.os, 'ubuntu')