Allow repo fork even when `shared_runners_enabled` conflicts with its group
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## Problem
An Ultimate SaaS customer reports that when they attempt to fork the [GitLab docker-machine](https://gitlab.com/gitlab-org/ci-cd/docker-machine) repo to their project, it fails because they do not have shared runners enabled on the project. This is very similar to https://gitlab.com/gitlab-org/gitlab/-/issues/276930.
I duplicated the behavior on a [namespace that has shared runners disabled](https://gitlab.com/gitlab-gold/mgibsongl). The network tab on the browser inspector reveals a `409` error when attempting to fork, and the error message is:
```
{"message":{"shared_runners_enabled":["cannot be enabled because parent group does not allow it"]}}
```
## Proposed solution
I believe the proposed solutions provided on the related issue I linked to above can probably apply here, as well.
* Keep project value if `Allow projects and subgroups to override the group` setting is set, as it already does
* Otherwise inherit group setting of disabled, if a project has enabled set
* Keep project setting of enabled/disabled, if group setting is enabled.
[Zendesk ticket](https://gitlab.zendesk.com/agent/tickets/296238) (internal)
issue