Resolve "Inconsistent schema - indexes"

What does this MR do and why?

In this MR, we are fixing the index_approval_rules_code_owners_rule_type index.

In our structure.sql file, the index seems fine, but the index in our production database is not correct.

structure.sql:

index_approval_rules_code_owners_rule_type btree (merge_request_id) WHERE (rule_type = 2);

production db:

index_approval_rules_code_owners_rule_type btree (merge_request_id, rule_type) WHERE rule_type = 2

Please take a look here to know more about this issue.

up:

== 20220128155814 FixApprovalRulesCodeOwnersRuleTypeIndex: migrating ==========
-- indexes(:approval_merge_request_rules)
   -> 0.0029s
-- rename_index(:approval_merge_request_rules, "index_approval_rules_code_owners_rule_type", "index_approval_rules_code_owners_rule_type_old")
   -> 0.0009s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:approval_merge_request_rules, :merge_request_id, {:where=>"rule_type = 2", :name=>"index_approval_rules_code_owners_rule_type", :algorithm=>:concurrently})
   -> 0.0028s
-- add_index(:approval_merge_request_rules, :merge_request_id, {:where=>"rule_type = 2", :name=>"index_approval_rules_code_owners_rule_type", :algorithm=>:concurrently})
   -> 0.0021s
-- transaction_open?()
   -> 0.0000s
-- indexes(:approval_merge_request_rules)
   -> 0.0076s
-- remove_index(:approval_merge_request_rules, {:algorithm=>:concurrently, :name=>"index_approval_rules_code_owners_rule_type_old"})
   -> 0.0016s
== 20220128155814 FixApprovalRulesCodeOwnersRuleTypeIndex: migrated (0.0214s) =

down:

== 20220128155814 FixApprovalRulesCodeOwnersRuleTypeIndex: reverting ==========
-- transaction_open?()
   -> 0.0000s
-- indexes(:approval_merge_request_rules)
   -> 0.0055s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- remove_index(:approval_merge_request_rules, {:algorithm=>:concurrently, :name=>"index_approval_rules_code_owners_rule_type"})
   -> 0.0021s
-- execute("RESET statement_timeout")
   -> 0.0007s
== 20220128155814 FixApprovalRulesCodeOwnersRuleTypeIndex: reverted (0.0110s) =

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Related to #349549 (closed)