(PA-5794) Update Checkout GitHub Action

Prior to this commit, the GitHub Actions were using the long end-of-life
Checkout action.

This commit updates Checkout to the latest version possible. The version
used in install_puppet.yaml is limited to v3 (which is end-of-life)
because it uses a version of NodeJS that relies on a compiler version
newer than CentOS 7 or Ubuntu 18.04 can support.

Once we drop support for those platforms, we should be able to use
Checkout v4 everywhere.

Author: mhashizume

.github/workflows/checks.yaml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout current PR
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
         with:
           fetch-depth: 0
       - name: Run shellcheck

.github/workflows/install_puppet.yaml

@@ -6,6 +6,8 @@ on:
   pull_request:
     branches: [ main ]
 
+# Even thought checkout@v3 is end-of-life, we cannot upgrade to v4
+# because it cannot run on CentOS 7 and Ubuntu 18.04
 jobs:
   install-puppet:
     name: ${{ matrix.collection }} / ${{ matrix.os.name }}
@@ -26,7 +28,7 @@ jobs:
       image: ${{ matrix.os.image }}
     steps:
       - name: Checkout current PR
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
         with:
           fetch-depth: 0
       - name: Install puppet-agent from ${{ matrix.collection }} collection