Adapt `merge-dependabot-reusable` to support RTC workflow

[ppkarwasz]

With the introduction of the Review-to-Commit process, the current merge-dependabot-reusable GitHub Actions workflow needs to be revised. The RTC policy introduces new constraints that directly impact how Dependabot PRs can be processed and merged.

Problems

1. Review requirement:
   The workflow can no longer merge PRs directly, as the RTC policy mandates at least one code review before merging.

2. Triggering required checks:
   Any commits made by the workflow (e.g., adding changelog files) must trigger all required status checks. This behavior is only guaranteed if the workflow uses a Personal Access Token (PAT) with appropriate permissions, instead of the default GITHUB_TOKEN.

3. Support for maintainers:
   To ease the additional manual steps introduced by RTC, the updated workflow should:

   • Handle PRs that update multiple dependencies at once (e.g., bundler mode).
   • Enable GitHub's auto-merge feature after making its changes, so the PR merges automatically once it receives a review and passes checks.

Propose solution

• Create a new reusable workflow (e.g., process-dependabot-reusable) that addresses these constraints.
• Ensure it uses a PAT to push changelog updates and re-run checks.
• Add logic to support multi-dependency updates and enable auto-merge.