chore: consolidate dependency management to Renovate-only

Replace Dependabot with Renovate for dependency version updates,
matching the approach already used in diracx-charts and diracx.

Author: chrisburr

.github/dependabot.yml

@@ -0,0 +1,12 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": ["config:recommended"],
+  "minimumReleaseAge": "7 days",
+  "packageRules": [
+    {
+      "description": "Group all GitHub Actions updates together",
+      "matchManagers": ["github-actions"],
+      "groupName": "GitHub Actions"
+    }
+  ]
+}