Create and synchronize a PR in a dependent repository

# Hacking

```sh
git clone https://code.forgejo.org/actions/setup-forgejo
export PATH=$(pwd)/setup-forgejo:$PATH
git clone https://code.forgejo.org/actions/cascading-pr
cd cascading-pr
forgejo-curl.sh logout
forgejo-runner.sh teardown
forgejo.sh teardown
forgejo.sh setup root admin1234 codeberg.org/forgejo/forgejo 1.21
forgejo-runner.sh setup
url=http://$(cat forgejo-ip):3000
firefox $url
tests/run.sh --debug
tests/run.sh --debug create_pull_request
cascading-pr.sh --debug --origin-url "$url" --origin-repo "user1/originrepo" --origin-token "$(cat /tmp/cascading-pr/user1/token)" --origin-pr 1 --destination-url "$url" --destination-repo "user2/destinationrepo" --destination-token "$(cat /tmp/cascading-pr/user2/token)" --destination-branch "main" --update "upgraded"
```