Move CI actions off deprecated Node 20 runtimes (#6)

Updates checkout and setup-node to their current Node 24 action runtimes and adds Dependabot monitoring for future GitHub Actions updates.

Author: JamieP-205

.github/dependabot.yml

@@ -5,3 +5,8 @@ updates:
     schedule:
       interval: monthly
     open-pull-requests-limit: 5
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: monthly
+    open-pull-requests-limit: 3

.github/workflows/ci.yml

@@ -15,8 +15,8 @@ jobs:
       run:
         working-directory: v2
     steps:
-      - uses: actions/checkout@v4
-      - uses: actions/setup-node@v4
+      - uses: actions/checkout@v7
+      - uses: actions/setup-node@v7
         with:
           node-version: 22
           cache: npm