Change project templates contribution flow
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### Problems
1. The current process of adding/updating of project templates is complicated. It requires user to have a GDK installed and then to manually execute a list of commands. It's complicated and inefficient, which makes it difficult for the community to contribute.
2. The list of [project templates](https://gitlab.com/gitlab-org/project-templates) is not in sync with [a built-in list of templates for new projects](https://gitlab.com/projects/new#create_from_template).
### Proposal
Currently, we create vendor archives to distribute the built-in templates. They should match what user sees in [project-template group](https://gitlab.com/gitlab-org/project-templates) but it's often not the case.
To fix that we can reverse the contribution flow. The SSOT will be the project-templates group. Projects listed there will be automatically archived and distributed with the GitLab package.
For automation, we can build a bot similar to [renovate-bot](https://gitlab.com/gitlab-org/frontend/renovate-gitlab-bot). The bot will:
1. Get a list of projects from the project-template group.
2. Detect changes between the template from project-template group and the vendored archive.
3. Open a MR to gitlab-org/gitlab with new changes.
#### The new flow
**User wants to create a new template**
1. The user creates a public template and requests a review from a GitLab team member
2. The GitLab team member approves it and forks the project into `project-templates` group
3. The bot automatically detect the new project and opens a merge request to gitlab-org/gitlab.
4. The GitLab team member verifies the merge request and merges it.
**User wants to update the existing template**
1. The user opens a merge request for project from `project-templates` group
2. The GitLab team member reviews, approves and merges it.
3. The bot automatically detect that the project was changed and opens a merge request to gitlab-org/gitlab.
4. The GitLab team member verifies the merge request and merges it.
issue