chore: open the release PR as a draft

[VascoSch92]

What

Add "draft-pull-request": true to release-please-config.json so this repo's own chore(main): release X.Y.Z PR opens as a draft.

Why

This adds a deliberate "ready to ship" gate: a draft PR can't be merged, and merging the release PR is what publishes the release. So someone has to explicitly mark it Ready for review before it can be merged — a useful checkpoint before cutting a release.

Notes

• draft-pull-request is a release-please config-file option (not an action input). The action runs in manifest mode and reads it from release-please-config.json, so this one line is all that's needed.
• Scope: this only affects this repo's own release PRs (dogfooding). Consumer repos each have their own config file and are unaffected.
• release-please won't flip the PR back to draft once a maintainer marks it Ready for review (it never issues a draft↔ready conversion on update).