try 16.x on Ubuntu only for now

- that one [passed](https://github.com/agilgur5/react-signature-canvas/actions/runs/12562533381/job/35023139677)
- 18.x and other 16.x versions failed to install `node-canvas`, c.f. #112 (comment)

.github/workflows/ci.yml

@@ -8,8 +8,9 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        node-version: [16.x, 18.x] # LTS Node: https://nodejs.org/en/about/releases/
-        os: [ubuntu-latest, windows-latest, macOS-latest]
+        # TODO: add back Node 18 once node-canvas is properly supported: https://github.com/Automattic/node-canvas/issues/2025
+        node-version: [16.x] # LTS Node: https://nodejs.org/en/about/releases/
+        os: [ubuntu-latest]
 
     steps:
     - name: Checkout repo