fix(actionCreators): Migrate mergeGroups to async/await and remove callbacks

[sentry]

This PR addresses the no-callback-api-request coding convention violation in static/app/actionCreators/group.tsx related to the mergeGroups function.

Problem:
The mergeGroups function was partially migrated to use api.requestPromise with async/await, but it still accepted and invoked options.success, options.error, and options.complete callbacks. This hybrid approach violated the coding convention which mandates promise-based error handling over callback patterns.

Solution:

1. static/app/actionCreators/group.tsx (mergeGroups):
   • Removed the options: RequestCallbacks parameter.
   • Modified the function to return the API response data directly upon success.
   • Ensured errors are re-thrown, allowing callers to handle them with try/catch.
2. static/app/views/issueList/actions/index.tsx (Call Site 1):
   • Removed the now-unnecessary empty {} third argument passed to mergeGroups.
   • Added a .catch(() => {}) to the mergeGroups call to prevent unhandled promise rejections for this fire-and-forget usage.
3. static/app/views/issueDetails/groupSimilarIssues/similarStackTrace/index.tsx (Call Site 2):
   • Replaced the new Promise((resolve, reject) => { mergeGroups(..., {success, error}) }) wrapper with a direct mergeGroups(api, {...}) call, leveraging the function's new promise-returning nature.

Note: The RequestCallbacks import in static/app/actionCreators/group.tsx has been retained as bulkDelete and bulkUpdate still utilize this callback pattern and will require separate migrations.

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Fixes CODING-CONVENTIONS-4KZ

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[ryan953]

dupe of #126268