Avoid executing .includes(...).order("ids.ordering").ids

Apparently, the `ids` method, which originally was implemented as
`pluck(:id)`, sometimes returned duplicate ids [1]. Fixing that issue
generated another issue [2] when combining `.includes` and `order`.
There was an attempt to fix it [3], but it still doesn't fix the case
where the ordering column belongs to an association [4].

This means that, when upgrading to Rails 7.1, calling `.ids` on
`budget.investments.includes(:heading).sort_by_random.for_render`
results in an invalid SQL statement:

```
ActiveRecord::StatementInvalid:
PG::GroupingError: ERROR:  column "ids.ordering" must appear in
the GROUP BY clause or be used in an aggregate function
  LINE 1: ... = $4 GROUP BY "budget_investments"."id" ORDER BY
  ids.orderi...
```

To solve it, we could once again use `.pluck(:id)` instead of `ids`, but
I'm not sure whether there would be a risk to get duplicate IDs in some
cases. We cannot call `.reorder` or `.unscope(:order)` either because we
need the IDs to be ordered randomly.

So we're removing the `includes(:heading)` part when getting the IDs.
Since using `includes` generates a separate query that doesn't affect
the query to get the IDs, removing it makes no difference.

Another option would be to remove the `for_render` call, since we're
including the headings to avoid the N+1 queries problem, but we're doing
so without benchmarks showing that it does actually make a difference.

[1] Issue 46455 in https://github.com/rails/rails
[2] Issue 48080 in https://github.com/rails/rails
[3] Pull request 48101 in https://github.com/rails/rails
[4] Issue 50263 in https://github.com/rails/rails

app/controllers/budgets/investments_controller.rb

@@ -38,7 +38,7 @@ module Budgets
 
     def index
       @investments = investments.page(params[:page]).per(PER_PAGE).for_render
-      @investment_ids = @investments.ids
+      @investment_ids = @investments.unscope(:includes).ids
 
       @investments_in_map = investments
       @tag_cloud = tag_cloud