Update actions and add dependabot with cooldown

Author: vcsjones

.github/dependabot.yml

@@ -0,0 +1,8 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    cooldown:
+      default-days: 7

.github/workflows/ruby.yml

@@ -14,9 +14,9 @@ jobs:
         ruby: [ '3.1', '3.2', '3.3', '3.4', '4.0' ]
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0
     - name: Set up Ruby ${{ matrix.ruby }}
-      uses: ruby/setup-ruby@d697be2f83c6234b20877c3b5eac7a7f342f0d0c
+      uses: ruby/setup-ruby@d45b1a4e94b71acab930e56e79c6aa188764e7f9
       with:
         ruby-version: ${{ matrix.ruby }}
     - name: Build and test