Add unique contraint for parent link work_item_id

What does this MR do and why?

Each work item can have only single parent, so work_item_id can occur in parent links table only once.

Related to #367951 (closed) Complementary model MR: !92513 (merged)

Screenshots or screen recordings

DB migration output:

honza@pc ~/gitlab-development-kit/gitlab (jp-single-child-db)$ rake db:migrate
main: == 20220715054506 AddParentLinkUniqueWorkItemIndex: migrating =================
main: -- transaction_open?()
main:    -> 0.0000s
main: -- index_exists?(:work_item_parent_links, :work_item_id, {:unique=>true, :name=>"index_work_item_parent_links_on_work_item_id", :algorithm=>:concurrently})
main:    -> 0.0039s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- add_index(:work_item_parent_links, :work_item_id, {:unique=>true, :name=>"index_work_item_parent_links_on_work_item_id", :algorithm=>:concurrently})
main:    -> 0.0121s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- indexes(:work_item_parent_links)
main:    -> 0.0023s
main: -- remove_index(:work_item_parent_links, {:algorithm=>:concurrently, :name=>"index_parent_links_on_work_item_id_and_work_item_parent_id"})
main:    -> 0.0035s
main: == 20220715054506 AddParentLinkUniqueWorkItemIndex: migrated (0.0294s) ========

ci: == 20220715054506 AddParentLinkUniqueWorkItemIndex: migrating =================
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- index_exists?(:work_item_parent_links, :work_item_id, {:unique=>true, :name=>"index_work_item_parent_links_on_work_item_id", :algorithm=>:concurrently})
ci:    -> 0.0029s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0002s
ci: -- add_index(:work_item_parent_links, :work_item_id, {:unique=>true, :name=>"index_work_item_parent_links_on_work_item_id", :algorithm=>:concurrently})
ci:    -> 0.0068s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0002s
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- indexes(:work_item_parent_links)
ci:    -> 0.0022s
ci: -- remove_index(:work_item_parent_links, {:algorithm=>:concurrently, :name=>"index_parent_links_on_work_item_id_and_work_item_parent_id"})
ci:    -> 0.0034s
ci: == 20220715054506 AddParentLinkUniqueWorkItemIndex: migrated (0.0194s) ========


honza@pc ~/gitlab-development-kit/gitlab (jp-single-child-db)$ rake db:rollback:main db:rollback:ci
main: == 20220715054506 AddParentLinkUniqueWorkItemIndex: reverting =================
main: -- transaction_open?()
main:    -> 0.0000s
main: -- index_exists?(:work_item_parent_links, [:work_item_id, :work_item_parent_id], {:name=>"index_parent_links_on_work_item_id_and_work_item_parent_id", :algorithm=>:concurrently})
main:    -> 0.0037s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- add_index(:work_item_parent_links, [:work_item_id, :work_item_parent_id], {:name=>"index_parent_links_on_work_item_id_and_work_item_parent_id", :algorithm=>:concurrently})
main:    -> 0.0141s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- indexes(:work_item_parent_links)
main:    -> 0.0023s
main: -- remove_index(:work_item_parent_links, {:algorithm=>:concurrently, :name=>"index_work_item_parent_links_on_work_item_id"})
main:    -> 0.0034s
main: == 20220715054506 AddParentLinkUniqueWorkItemIndex: reverted (0.0310s) ========

ci: == 20220715054506 AddParentLinkUniqueWorkItemIndex: reverting =================
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- index_exists?(:work_item_parent_links, [:work_item_id, :work_item_parent_id], {:name=>"index_parent_links_on_work_item_id_and_work_item_parent_id", :algorithm=>:concurrently})
ci:    -> 0.0031s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0003s
ci: -- add_index(:work_item_parent_links, [:work_item_id, :work_item_parent_id], {:name=>"index_parent_links_on_work_item_id_and_work_item_parent_id", :algorithm=>:concurrently})
ci:    -> 0.0167s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0004s
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- indexes(:work_item_parent_links)
ci:    -> 0.0026s
ci: -- remove_index(:work_item_parent_links, {:algorithm=>:concurrently, :name=>"index_work_item_parent_links_on_work_item_id"})
ci:    -> 0.0037s
ci: == 20220715054506 AddParentLinkUniqueWorkItemIndex: reverted (0.0314s) ========

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