Fix create-release-pr workflow

[soranakahara]

This PR fixes several issues found after testing the workflow introduced in the previous PR (#31).

• PRs were not categorized by labels
  → Added a configuration file at .github/release.yml.
  See: https://docs.github.com/en/repositories/releasing-projects-on-github/automatically-generated-release-notes#configuring-automatically-generated-release-notes

• Generated release notes included all PRs in the repository instead of only those since the previous release
  → Added previous_tag_name and target_commitish parameters to the generate-notes API call.

• Release PRs created by GitHub Actions could not be merged due to branch protection rules
  → Added the ready_for_review activity type to the build workflow trigger.
  → Release PRs are now created as draft PRs.
  → When maintainers mark the PR as Ready for review, the build workflow is automatically triggered and the PR becomes mergeable once checks pass.

Note

Just Specifying

on: [pull_request]

means

types: [opened, synchronize, reopened]

See https://docs.github.com/en/actions/reference/workflows-and-actions/events-that-trigger-workflows#pull_request