Guard legacy runner status in graphql by feature flag

What does this MR do and why?

We want to remove legacy status from graphql in %16.0, this MR adds a feature flag to make the feature removal more controlled.

I don't plan to "rollout" it, just 100% enable in one day, and then we'll shortly remove it in Remove legacy mode for runnner graphql status (!117886 - merged)

How to set up and validate locally

1. create runner

2. pause it

3. run the following query in the : /-/graphql-explorer

   query {
     project(fullPath: "root/main") {
       id
       fullPath
       runners {
         nodes {
           status(legacyMode: "14.5")
         }
       }
     }
   }

   You will get:

   {
     "data": {
       "project": {
         "id": "gid://gitlab/Project/20",
         "fullPath": "root/main",
         "runners": {
           "nodes": [
             {
               "status": "NEVER_CONTACTED"
             },
             {
               "status": "PAUSED"
             }
           ]
         }
       }
     }
   }

4. Enable FF

   Feature.enable(:disable_runner_graphql_legacy_mode)

5. Run the query again and get:

   {
     "data": {
       "project": {
         "id": "gid://gitlab/Project/20",
         "fullPath": "root/main",
         "runners": {
           "nodes": [
             {
               "status": "NEVER_CONTACTED"
             },
             {
               "status": "ONLINE"
             }
           ]
         }
       }
     }
   }

6. Click the invite members button.

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