Remove legacy mode for runnner graphql status

What does this MR do and why?

Following up from Guard legacy runner status in graphql by featur... (!117875 - merged), this MR removes all the code associated with legacyMode for runner status.

See more in GraphQL API: Runner status will not return "pau... (#344648 - closed)

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'll see

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

MR acceptance checklist

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