feat: automerge PRs

**Is your feature request related to a problem? Please describe.**
I am using the backport action with Forgejo and it works very nicely. The repository settings require 1. CI to run successfully, and 2. at least one approval. We also use renovate and they have a useful feature of scheduling to automerge when all conditions are fulfilled. Would be a nice addition to have this for backporting, too.

**Describe the solution you'd like**
Add a parameter `automerge` or similar that, if true, sets the PR to automerge. Should likelely be disabled by default.

**Describe alternatives you've considered**
- keep doing merges manually
- add a step in the job in CI that performs this operation

**Additional context**
Renovate's doc on platformAutomerge option: https://docs.renovatebot.com/configuration-options/#platformautomerge

Probably, this feature also needs a parameter like [automergeStrategy](https://docs.renovatebot.com/configuration-options/#automergestrategy), because that must be specified in the POST request, e.g., for Forgejo: https://codeberg.org/api/swagger#/repository/repoMergePullRequest

Start by locating the backport action's pull-request creation flow and its Forgejo integration, then review how action parameters are defined and passed to platform APIs. Compare the required Forgejo merge request fields with the Renovate platformAutomerge and automergeStrategy behavior. Done means an opt-in automerge setting works with the required strategy while preserving the default manual-merge behavior.

Written by the indexing model from the issue text.