Upgrade to GitHub-native Dependabot

Author: dependabot-preview[bot]

.github/dependabot.yml

@@ -0,0 +1,15 @@
+version: 2
+updates:
+- package-ecosystem: composer
+  directory: "/"
+  schedule:
+    interval: monthly
+  open-pull-requests-limit: 10
+  versioning-strategy: increase
+  ignore:
+  - dependency-name: symfony/polyfill-php80
+    versions:
+    - 1.22.0
+  - dependency-name: phpunit/phpunit
+    versions:
+    - 8.5.14