fix(ci): Upgrade Node from 16.x to 18.x

16.x reached end of life in September and our dependencies no longer
support it.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

.github/workflows/ci.yaml

@@ -2,8 +2,8 @@ name: verify
 
 on:
   push:
-    branches: [master]
-  pull_request_target:
+    branches: [main]
+  pull_request:
 
 jobs:
   all:
@@ -15,7 +15,7 @@ jobs:
       - uses: actions/setup-node@v2
         with:
           cache: yarn
-          node-version: 16.x
+          node-version: 18.x
 
       - name: Install
         run: yarn install --frozen-lockfile

.nvmrc

@@ -1 +1 @@
-16.13.0
+18.20.3