Disable Remove button when gem has only one confirmed owner

The Remove button on the owners page was clickable even when the
gem had only one owner. The server rejected the request silently.
Now the button is disabled when there's only one confirmed ownership,
preventing the confusing no-op.

Fixes #5666

Author: tmchow

app/views/owners/_owners_table.html.erb

@@ -57,6 +57,7 @@
         <%= button_to t("remove"),
           rubygem_owner_path(@rubygem.slug, ownership.user.display_id),
           method: "delete",
+          disabled: @ownerships.select(&:confirmed?).count <= 1,
           data: { confirm: t("owners.index.confirm_remove") },
           class: "form__submit form__submit--small" %>
         <% end %>