Prepares confidential notes index

What does this MR do and why?

In an upcoming migration we will need to iterate through confidential notes in batches by ID, so we need an index on its ID.

Although there is already an index on confidential column, this is not so useful for the upcoming migration - related to !87908 (comment 975301306)

Screenshots or screen recordings

Migration output:

$ rake db:migrate
ci: == 20220608114734 PrepareConfidentialNoteIndexOnId: migrating =================
ci: -- index_exists?(:notes, :id, {:where=>"confidential = true", :name=>"index_notes_on_id_where_confidential", :algorithm=>:concurrently})
ci:    -> 0.0071s
ci: -- add_index_options(:notes, :id, {:where=>"confidential = true", :name=>"index_notes_on_id_where_confidential", :algorithm=>:concurrently})
ci:    -> 0.0002s
ci: == 20220608114734 PrepareConfidentialNoteIndexOnId: migrated (0.0112s) ========

$ rake db:rollback:main
main: == 20220608114734 PrepareConfidentialNoteIndexOnId: reverting =================
main: == 20220608114734 PrepareConfidentialNoteIndexOnId: reverted (0.0107s) ========

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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