ci(validate): drops support for node 16 (#4)

This PR drops support for Node 16, which is at EOL. It also extracts all
setup for script-running actions into a composite action so that I don't
have to maintain things in multiple places.

.github/composite-workflows/pnpm-script-base/action.yml

@@ -0,0 +1,21 @@
+name: Base Pnpm Script Action
+description: Setup everything to run a pnpm script
+inputs:
+  node-version:
+    description: the version of Node to use
+    required: true
+
+runs:
+  using: composite
+  steps:
+    - name: Install dependencies
+      uses: pnpm/action-setup@v2
+      with:
+        version: 8
+        run_install: |
+          - recursive: true
+    - name: Setup Node.js
+      uses: actions/setup-node@v3
+      with:
+        cache: "pnpm"
+        node-version: ${{ matrix.node-version }}

.github/workflows/validate.yml

@@ -1,6 +1,6 @@
 name: CI
 on:
-  push:
+  pull_request:
     branches:
       - release/v*
       - main
@@ -11,59 +11,38 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: ["16.x", "18.x", "20.x"]
+        node-version: ["18.x", "20.x"]
     steps:
       - name: Checkout
         uses: actions/checkout@v4
-      - name: Install dependencies
-        uses: pnpm/action-setup@v2
+      - name: Setup Script Runner
+        uses: ./.github/composite-workflows/pnpm-script-base
         with:
-          version: 8
-          run_install: |
-            - recursive: true
-      - name: Setup Node.js
-        uses: actions/setup-node@v3
-        with:
-          cache: "pnpm"
           node-version: ${{ matrix.node-version }}
       - run: pnpm run lint
   test:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: ["16.x", "18.x", "20.x"]
+        node-version: ["18.x", "20.x"]
     steps:
       - name: Checkout
         uses: actions/checkout@v4
-      - name: Install dependencies
-        uses: pnpm/action-setup@v2
-        with:
-          version: 8
-          run_install: |
-            - recursive: true
-      - name: Setup Node.js
-        uses: actions/setup-node@v3
+      - name: Setup Script Runner
+        uses: ./.github/composite-workflows/pnpm-script-base
         with:
-          cache: "pnpm"
           node-version: ${{ matrix.node-version }}
       - run: pnpm test
   compile:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: ["16.x", "18.x", "20.x"]
+        node-version: ["18.x", "20.x"]
     steps:
       - name: Checkout
         uses: actions/checkout@v4
-      - name: Install dependencies
-        uses: pnpm/action-setup@v2
-        with:
-          version: 8
-          run_install: |
-            - recursive: true
-      - name: Setup Node.js
-        uses: actions/setup-node@v3
+      - name: Setup Script Runner
+        uses: ./.github/composite-workflows/pnpm-script-base
         with:
-          cache: "pnpm"
           node-version: ${{ matrix.node-version }}
       - run: pnpm run compile