chore: remove dependabot.yml — we use Renovate for dependency management

This repo already has renovate.json configured. The dependabot.yml was
added in error and creates conflicting dependency update PRs. Removing
it to consolidate on Renovate as the single dependency management tool.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/dependabot.yml

@@ -1,12 +0,0 @@
-version: 2
-updates:
-  - package-ecosystem: "github-actions"
-    directory: "/"
-    schedule:
-      interval: "weekly"
-      day: "monday"
-    open-pull-requests-limit: 5
-    labels:
-      - "dependencies"
-      - "ci"
-