(maint) Update Checkout GitHub Action

This commit updates the Checkout GitHub Action from 2 to 3, as
version 2 is deprecated because it uses Node.js 12. See:

https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/

Author: mhashizume

.github/workflows/checks.yaml

@@ -20,7 +20,7 @@ jobs:
     runs-on: ${{ matrix.cfg.os }}
     steps:
       - name: Checkout current PR
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
         with:
           fetch-depth: 0
 

.github/workflows/rspec_tests.yaml

@@ -26,7 +26,7 @@ jobs:
     runs-on: ${{ matrix.cfg.os }}
     steps:
       - name: Checkout current PR
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Install ruby version ${{ matrix.cfg.ruby }}
         uses: ruby/setup-ruby@v1

.github/workflows/snyk_monitor.yaml

@@ -11,7 +11,7 @@ jobs:
     name: Snyk Monitor
     steps:
       - name: Checkout current PR
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
       - name: Setup Ruby
         uses: ruby/setup-ruby@v1
         with: