Update policy project selection for groups

What does this MR do and why?

Update policy project selection for groups

• rename projectPath to namespacePath to be more generic
• update tests

Related to #345316 (closed)

Screenshots or screen recordings

No visual changes

Page	Screenshot
Viewing a project's security policy project
Viewing a group's security policy project

I verified these changes with the following steps:

• linked/unlinked a policy project on a project
• linked/unlinked a policy project on a group

How to set up and validate locally

1. Enable :group_level_security_policies feature flag
2. Upload a GitLab Ultimate license
3. Navigate to a group => Security & Compliance => Policies and modify the linked security project
4. Navigate to a project => Security & Compliance => Policies and modify the linked security project

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.