Create notification for existing forks

[zkamvar]

When a lesson transitions, forks will no longer be in synch and must be deleted.

Not everyone who has a fork is aware that the lesson transition will happen and it would prevent a lot of chaos if we made them aware of this (note: might be a bit too late for the lessons that are in the beta phase, but there's not much we can do about this now)

There are a few problems to this, however:

1. not all forks have issues, so we would need to open pull requests to alert the repository owners or use a different means
2. not everyone who has a fork will remember that they have a fork
3. some forks are used as a modified version of the lesson and transitioning would break the lesson history.

1 and 2 have good technical solutions (i.e. pull requests will notify the owners well enough and the owners in situation 2 might actually be happy for the reminder). Number 3 is more rare, but definitely a feature of the community.

Process

We can use the GitHub API with @carpentries-bot to deliver the messages. We will do the following:

1. for each repository in our official lessons, we run GET /repos/{carpentries_org}/{lesson}/pulls to extract all the pull requests
2. for each pull request, we check the values of has_issues or has_discussions and the decide the next steps from there

Forks with issues

3. use POST /repos/{carpentries_org}/{lesson}/issues to create an issue linking them to documentation.

Forks without issues

For forks without issues, we need to create a pull request, and to create a pull request, we need to create a fork, but the problem is, you can't have more than one fork at a time, thus, we will need to take a different approach.

I'm not sure if this would work, but it is worth a shot: Instead of creating a separate fork for each fork, create a repository owned by @carpentries-bot (called workbench-transition-helper) and then use the strategy for updating the styles repository:

3. add the fork as a unique remote in workbench-transition-helper
4. fetch the default branch into a new ref called ({owner}-{repo})
5. push that ref up to workbench-transition-helper
6. use POST /repos/{carpentries_org}/{lesson}/pulls with head = carpentries-bot:{owner}-{repo} to create the pull request.

If this is done with enough time ahead of time, then we can circumvent some chaos.