Upgrade to actions/checkout version 3 and setup dependabot.

I certainly won't regret this commit.

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"

.github/workflows/main.yml

@@ -38,7 +38,7 @@ jobs:
           sudo apt-get install -y texlive-fonts-recommended || true
 
       - name: Cache ImageMagick
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         env:
           cache-name: cache-ImageMagick
         with: