chore: Update deprecated GitHub Actions to v4

- Update actions/checkout@v3 → v4 in validate_examples.yml
- Update actions/upload-artifact@v3 → v4 in validate_examples.yml
- Update codecov/codecov-action@v3 → v4 in ci.yml

These v3 actions are deprecated and cause CI warnings/failures.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dshkol

.github/workflows/ci.yml

@@ -40,7 +40,7 @@ jobs:
         pytest --cov=pycancensus --cov-report=xml
     
     - name: Upload coverage to Codecov
-      uses: codecov/codecov-action@v3
+      uses: codecov/codecov-action@v4
       with:
         file: ./coverage.xml
         fail_ci_if_error: false

.github/workflows/validate_examples.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
 
     - name: Set up Python
       uses: actions/setup-python@v4
@@ -60,7 +60,7 @@ jobs:
 
     - name: Upload validation report
       if: always()
-      uses: actions/upload-artifact@v3
+      uses: actions/upload-artifact@v4
       with:
         name: validation-report
         path: validation_output.txt