ci: drop Node 18 from matrix to match engines >=20

serialize-javascript@7.0.5 (the patched version from the protobufjs/
follow-redirects override in this PR) uses globalThis.crypto, which
is only a global on Node 19+. Commit 4b1b79a bumped engines to >=20
but left the CI matrix on 18.x/20.x/22.x. Aligning the matrix with
engines.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: andrewjaykeller

.github/workflows/node.js.yml

@@ -16,7 +16,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [18.x, 20.x, 22.x]
+        node-version: [20.x, 22.x]
         # See supported Node.js release schedule at https://nodejs.org/en/about/releases/
 
     steps: