Learn GitLab should no longer be created as its own project

Context: Our longer term onboarding and activation strategy no longer requires Learn GitLab to be separate project. While we still want to keep Learn GitLab we do not need a separate project to be created as part of registration.

PM tasks:

• Confirm expected behaviour with @s_awezec and @gdoud
  • According to this comment this behaviour is believed to be a result of missing a part of clean up of this experiment: #337965 (closed)
• If decided that this behaviour is no longer expected, prioritize this clean up issue.
• Confirm the current behaviour of the onboarding tasks within Learn GitLab and ensure links do not break if Learn GitLab is no longer its own project

Current behaviour of onboarding tasks

Click to expand

Learn GitLab link	Directs to:
Invite your colleagues	✅
Create a repository	✅ If a README is created as part of registration this step is completed by default. If not, the link directs to the first project crated as part of registration.
Set up your first project's CI/CD	❌ Currently links to an Issue within the Learn GitLab Project -- needs to be updated #390257 (closed)
Start a free trial of GitLab Ultimate	✅
Add code owners	✅ https://docs.gitlab.com/ee/user/project/code_owners.html#set-up-code-owners
Enable require merge approvals	✅ https://docs.gitlab.com/ee/ci/pipelines/settings.html#coverage-check-approval-rule
Create an issue	✅ Links to Issues > List for the project created as part of registration
Submit a merge request (MR)	✅ Links to Merge Requests for the project created as part of registration
Run a Security scan using CI/CD	✅ Links to Security & Compliance > Configuration for the project created as part of registration

Plan

• Move 'onboarding' to only be created on the group that a user creates during registration. - !111622 (merged)
  • this will stop current onboarding that is happening on every group creation.
• Delete any onboarding_progresses that do not have a learn gitlab project for the group as they will be the errant creations outside of registration. - !112229 (merged),!112622 (merged)
  • this is needed so we don't break the currently correctly onboarded namespaces(by looking to see if user has a learn gitlab project) and ignore/not show the Learn GitLab experience for those w/o the learn gitlab project(this is our anchor into the past since learn gitlab is our trigger to know if the user was from registration).
• Once the errant onboarded groups are removed from onboarding_progresses table, we can safely change the logic to -> if a group is onboarded then show learn gitlab experience on the projects. - !111324 (merged)
• Remove Learn GitLab project creation. - !111324 (merged)
• Remove Learn GitLab background worker class. - !113899 (merged)