ci: refactor bump downstream versions worflow and adds demo-scene

[killme2008]

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What's changed and what's your intention?

This PR reactor the scripts to automatically trigger workflows in various repos based on version type, streamlining release processes and reducing manual effort.

Depends on GreptimeTeam/demo-scene#85

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

[Copilot]

Pull Request Overview

This PR refactors the release workflow to simplify version bumping across multiple downstream repositories and integrates triggering for a new demo-scene.

• Replaces the bump-doc-version job with bump-downstream-repo-versions.
• Updates the working directory command and environment variables for the new job.

Comments suppressed due to low confidence (1)

.github/workflows/release.yml:463

• [nitpick] The TARGET_REPOS variable uses 'demo' while the PR title mentions 'demo-scene'. Consider aligning the naming for consistency.

TARGET_REPOS: website,docs,demo

[daviderli614]

We can set Repository variables so that we can modify the repository list at any time later.

env:
  DOWNSTREAM_TARGET_REPOS: ${{ vars.DOWNSTREAM_TARGET_REPOS }}

key: DOWNSTREAM_TARGET_REPOS
value: website,docs,demo