Allow updating the shared runners setting via GraphQL

What does this MR do and why?

This change allows users that belong to a custom role with the admin_runners permission enabled to be able to execute the Mutation.groupUpdate GraphQL Mutation in order to change the shared_runners_setting on the target group.

#442851

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Before	After

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(:custom_ability_admin_runners)

2. Visit the GraphQL Explorer at http://127.0.0.1:3000-/graphql-explorer
3. Execute the Mutation.groupUpdate Mutation to change the sharedRunnersSetting

mutation updateGroup($fullPath: ID!){
  groupUpdate(input: {fullPath: $fullPath, sharedRunnersSetting: DISABLED_AND_OVERRIDABLE}) {
    errors
    group{
      id
      sharedRunnersSetting
    }
  }
}