chore: add GitHub Actions workflow to auto-update CHANGELOG.md via calens (#41572)

Integrates the owncloud/reusable-workflows calens.yml reusable workflow
so that every push to master triggers changelog regeneration and opens
(or updates) a PR with the updated CHANGELOG.md.

Signed-off-by: Thomas Müller <1005065+DeepDiver1975@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: DeepDiver1975

.github/workflows/changelog.yml

@@ -0,0 +1,16 @@
+name: Changelog
+
+on:
+  push:
+    branches:
+      - master
+
+permissions:
+  contents: read
+
+jobs:
+  changelog:
+    uses: owncloud/reusable-workflows/.github/workflows/calens.yml@main
+    secrets:
+      APP_ID: ${{ secrets.CHANGELOG_APP_ID }}
+      APP_PRIVATE_KEY: ${{ secrets.CHANGELOG_APP_PRIVATE_KEY }}