Add GraphQL query to dashboard issues refactor

What does this MR do and why?

This MR adds a top-level issues GraphQL query to the dashboard issues refactor.

Before:

• The page just showed an empty state

After:

• The page now fetches all issues via GraphQL
• The user can click Next and Prev to go forward and backward through the issues list
• The user can click the Open, Closed, and All tabs to see the opened, closed, and all issues respectively

No changelog required, as it is behind feature flags vue_issues_dashboard and root_level_issues_query, default disabled

Follow-up MRs will add the ability to sort and filter the list.

Screenshots or screen recordings

FOSS	EE

How to set up and validate locally

1. In rails console enable the feature flags

   Feature.enable(:vue_issues_dashboard)
   Feature.enable(:root_level_issues_query)

2. Visit the issues dashboard page http://127.0.0.1:3000/dashboard/issues

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #377514 (closed)