Allow agent CI/CD access sharing within a personal namespace
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### Release notes
GitLab users targeting Kubernetes migrate to the agent for Kubernetes to use a more secure solution for managing their clusters from GitLab CI/CD. You can use a single agent from multiple projects and groups by sharing access to the agent connection. Previously, sharing agent access across projects was unavailable for personal namespaces. The current release expands CI/CD connection sharing to personal namespaces.
You can now use a single agent from any of your projects under a personal namespace.
### Problem to solve
Right now, the agent sharing CI/CD access and the consuming project need to be in the same group for access to work. Many users expect this to also work for personal namespaces.
### Proposal
- Allow CI/CD access sharing between projects in the same personal namespace. This is a matter of slightly tweaking [this check](https://gitlab.com/gitlab-org/gitlab/-/blob/58b61e5513553df9c5e63c37e8db9d09ab364632/app/services/clusters/agents/refresh_authorization_service.rb#L86).
- Update documentation!
### Discussion
* **Pro:** This would reduce friction for users evaluating the CI/CD tunnel, allowing them to do so in their personal namespace.
* **Con:** This could be confusing on GitLab.com, as personal namespaces seem to always be on the Free plan, limiting access to GitOps and Impersonation features.
### Intended users
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