ci: add nodejs v20 to test matrix

Author: antongolub

.github/workflows/ci.yaml

@@ -154,7 +154,7 @@ jobs:
     strategy:
       matrix:
         os: [ ubuntu-22.04 ]
-        node-version: [ 18 ]
+        node-version: [ 18, 20 ]
     name: Test (Node v${{ matrix.node-version }}, OS ${{ matrix.os }})
     runs-on: ${{ matrix.os }}
     steps: