GraphQL: Rename type argument in RunnersExportUsage mutation

What does this MR do and why?

This MR renames the type argument in the RunnersExportUsage mutation to runnerType. The mutation is in alpha, so we're good to change the name. There will be a short period of time during the rollout where the frontend might be out-of-sync with the backend, but given that our admins don't use this functionality in .com, it will have no real-world impact.

EE: true

MR acceptance checklist

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How to set up and validate locally

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

1. Ensure you have an Ultimate license associated with your GDK

2. Follow https://docs.gitlab.com/ee/development/database/clickhouse/clickhouse_within_gitlab.html#gdk-setup up to and including running migrations (bundle exec rake gitlab:clickhouse:migrate)

3. Run mutation in http://gdk.test:3000/-/graphql-explorer:

   Query:

   mutation runnersExportUsage($input: RunnersExportUsageInput!) {
     runnersExportUsage(input: $input) {
       errors
     }
   }

   Query variables:

   {
     "input": {
       "runnerType": "INSTANCE_TYPE",
       "fromDate": "2024-01-01",
       "toDate": "2024-06-05"
     }
   }