Redirect to "Most starred" tab when clicking the "Explore public projects" button in Projects Onboarding View

This issue is not yet ready for development. See &18493 (comment 2633355547) for context.

When logging in to GitLab for the first time. The /dashboard/projects page displays an onboarding view. This contains an "Explore public projects" button that redirects to the "Explore Projects > Trending" tab.

As part of Retire the Trending tab from Explore > Projects (#553408 - closed), we need to update this such that it redirects to "Explore projects > Most starred".

Implementation Guide

Note: This must be behind the retire_trending_projects feature flag.

1. Update _blank_state_welcome.html.haml to use explore_root_path.