Fix issue where user could not assign runner to a project

What does this MR do and why?

Fix issue where user could not assign runner to a project

Fixes #480459 (closed)

Changelog: fixed

MR acceptance checklist

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Screenshots or screen recordings

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n/a

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. In Project A settings, create a new Project runner.
2. In the Admin, CI/CD, Runners UI, edit the Project runner and unmap it from Project A via the Disable button.
3. Now try to map it to another project via the Enable button - it should succeed.