Migrate admin_index/admin_index_component.rb to use ViewComponent instead of mountain_view

[lenikadali]

Places where the component is used:

app/views/admin/supporters/index.html.erb
1:<%= render_component "admin_index",

app/views/admin/collections/index.html.erb
3:<%= render_component "admin_index",

Note on implementation: based on our experience in this PR no specs should break and all existing logic from the component should be preserved and where necessary, re-interpreted to be compatible with the ViewComponent API

_{Huly®: PC-2652}