Add Dependabot auto-merge workflow and migration script (#466)

## Summary

- Adds auto-merge workflow for Dependabot PRs using
`frequenz-floss/dependabot-auto-approve` action
- Includes migration script to create workflow file and disable
CODEOWNERS review requirement via GitHub API
- Splits changes into two commits: reset migration script to template,
then add new migration steps

## Changes

**First commit:** Reset `cookiecutter/migrate.py` to template, removing
old migration steps

**Second commit:**
- Add `create_dependabot_auto_merge_workflow()` function
- Add `disable_codeowners_review_requirement()` function to update
GitHub rulesets
- Include workflow template in cookiecutter
- Regenerate golden test files

Author: Marenz

.github/workflows/auto-dependabot.yaml

@@ -0,0 +1,22 @@
+name: Auto-merge Dependabot PR
+
+on:
+  pull_request:
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  auto-merge:
+    if: github.actor == 'dependabot[bot]'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Auto-merge Dependabot PR
+        uses: frequenz-floss/dependabot-auto-approve@3cad5f42e79296505473325ac6636be897c8b8a1 # v1.3.2
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          dependency-type: 'all'
+          auto-merge: 'true'
+          merge-method: 'merge'
+          add-label: 'tool:auto-merged'

RELEASE_NOTES.md

@@ -29,6 +29,8 @@ But you might still need to adapt your code:
 - New warning ignores for protobuf gencode versions in pytest.
 - mkdocstrings: Updated the deprecated `import` config key to `inventories` in `mkdocs.yml`.
 - Dependencies have been updated.
+- Added Dependabot auto-merge workflow using `frequenz-floss/dependabot-auto-approve` action.
+- Migration script now creates auto-merge workflow and disables CODEOWNERS review requirement via GitHub API.
 
 ## Bug Fixes