Enable dependabot, remove deprecated GH actions code

Author: Seldaek

.github/dependabot.yml

@@ -0,0 +1,7 @@
+version: 2
+
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"

.github/workflows/continuous-integration.yml

@@ -45,7 +45,7 @@ jobs:
 
       - name: Get composer cache directory
         id: composercache
-        run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+        run: "echo \"dir=$(composer config cache-files-dir)\" >> $GITHUB_OUTPUT"
 
       - name: Cache dependencies
         uses: actions/cache@v2

.github/workflows/phpstan.yml

@@ -34,7 +34,7 @@ jobs:
 
       - name: Get composer cache directory
         id: composercache
-        run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+        run: "echo \"dir=$(composer config cache-files-dir)\" >> $GITHUB_OUTPUT"
 
       - name: Cache dependencies
         uses: actions/cache@v2