Custom group and instance level project templates retain issues and merge requests
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### Summary
Custom group and instance level project templates retain open/closed issues and merge requests from the source template project when creating a new project.
This is related to #351032, which addressed commit history and pipelines being retained. However, issues and merge requests are also copied to the new project, which is inconsistent with the expected behavior of a "clean" project template.
### Current behavior
When creating a project from a custom group or instance level template:
- All issues (open and closed) from the template project are copied to the new project
- All merge requests (open, closed, and merged) from the template project are copied to the new project
### Expected behavior
Custom group and instance level project templates should act consistently with built-in templates and **not** import issues or merge requests from the source template project. A new project created from a template should start with a clean issue tracker and no merge requests.
### Proposal
Exclude issues and merge requests when creating projects from custom group or instance level templates, similar to how !226117 excluded commit history and !225088 excluded CI pipelines.
issue