ci: bump codecov-action to v4 (#9293)

## Description

v3 uses Node 16 which is EOL.
<img width="1124" alt="Screenshot 2024-04-26 at 10 24 23 AM"
src="https://github.com/Agoric/agoric-sdk/assets/21505/126d74b8-20df-4bd7-b41a-074d2ff11d63">


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### Security Considerations

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.github/actions/post-test/action.yml

@@ -48,7 +48,7 @@ runs:
         fi
     - name: Upload coverage reports to Codecov
       if: ${{ github.repository_owner == 'agoric' }} && ${{ steps.coverage-prep.outputs.skip == 'false' }} && (success() || failure())
-      uses: codecov/codecov-action@v3
+      uses: codecov/codecov-action@v4
       continue-on-error: true
       with:
         token: ${{ inputs.codecov-token }}