Support `#last` on relations with keyset order

What does this MR do and why?

When a relation has a complex order clause, ActiveRecord cannot reverse it automatically. But when the order clause is built using Gitlab::Pagination::Keyset::Order, we can determine the reverse order because we pass it in explicitly in these cases.

This patch allows ActiveRecord to use the specified reverse order when calling #last and other related methods that need a reverse order.

Before:

[1] pry(main)> Issue.order_by_relative_position.last
ActiveRecord::IrreversibleOrderError: Order "\"issues\".\"relative_position\" ASC NULLS LAST, \"issues\".\"id\" ASC" cannot be reversed automatically
from /Users/work/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/activerecord-6.1.6.1/lib/active_record/relation/query_methods.rb:1348:in `block in reverse_sql_order'

After:

[1] pry(main)> Issue.order_by_relative_position.last
=> #<Issue id:526 gitlab-org/gitlab-test#34>

MR acceptance checklist

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