GraphQL: Add NamespaceCiCdSettingsUpdate mutation

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR adds a NamespaceCiCdSettingsUpdate GraphQL mutation so that we can set the namespace_ci_cd_settings.allow_stale_runner_pruning value (EE-only) for a given namespace. It is inspired from the existing CiCdSettingsUpdate mutation (which works on projects). I'm handling the renaming of CiCdSettingsUpdate to ProjectCiCdSettingsUpdate in a follow-up deprecation MR.

Part of Implement worker to remove stale runners from G... (#361112 - closed)

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How to set up and validate locally

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Query

mutation namespaceCiCdSettingsUpdate($input: NamespaceCiCdSettingsUpdateInput!) {
  namespaceCiCdSettingsUpdate(input: $input) {
    ciCdSettings {
      allowStaleRunnerPruning
    }
    errors
  }
}

Variables

{
  "input": {
  	"fullPath": "flightjs",
  	"allowStaleRunnerPruning": true
	}
}

Results

{
  "data": {
    "namespaceCiCdSettingsUpdate": {
      "ciCdSettings": {
        "allowStaleRunnerPruning": true
      },
      "errors": []
    }
  }
}

After running the query, you should be able to run the following command in the GDK console to check that the GraphQL mutation had the intended effect:

> Group.find_by_full_path('flightjs').allow_stale_runner_pruning?
=> true

MR acceptance checklist

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