Resolve "Pause many runners at once"

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What does this MR do and why?

This MR adds a mutation allowing setting the paused status of a group of up to 50 runners in one go.

References

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Related to #376363 (closed)

MR acceptance checklist

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Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Scenario	GraphQL	UI
Pause
Un-pause

How to set up and validate locally

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

1. Ensure that you have at least one CI runner registered against the GDK instance

2. Navigate to http://gdk.test:3000/-/graphql-explorer and run the following mutation:

   mutation runnerBulkPause($input: RunnerBulkPauseInput!) {
     runnerBulkPause(input: $input) {
       errors
       updatedCount
       updatedRunners {
         id
         paused
       }
     }
   }

   Arguments:

   {
     "input": {
       "ids": [
         "gid://gitlab/Ci::Runner/1200",
         "gid://gitlab/Ci::Runner/1201"
       ],
       "paused": true
     }
   }