test: Remove support for Node 18 (#412)

* test: Add Node 20 to CI matrix

* test: Remove support for Node 18

.github/workflows/ci.yml

@@ -7,9 +7,6 @@ jobs:
     if: "!(github.actor == 'github-actions[bot]') "
     name: Build
     runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        node: [18, 20]
 
     steps:
     - name: Checkout
@@ -23,7 +20,7 @@ jobs:
     - name: Setup Nodejs
       uses: actions/setup-node@v4
       with:
-        node-version: ${{ matrix.node }}
+        node-version-file: '.nvmrc'
     # lerna expects to be authenticated for publishing to NPM. This step will fail CI if NPM is not
     # authenticated, even though this build does _not_ attempt to publish, as an extra check before merge
     # that Lerna is set to publish.