Resolve "Inconsistent schema - indexes"

What does this MR do and why?

We already have index_on_projects_path index on production, but we could not find it in structure.sql and migrations. In this MR, we are adding a new migration to add this index (if not exist).

https://thanos-query.ops.gitlab.net/graph?g0.expr=pg_stat_user_indexes_idx_scan%7Bindexrelname%3D%22index_on_projects_path%22%2C%20env%3D%22gstg%22%7D&g0.tab=0&g0.stacked=0&g0.range_input=2d&g0.max_source_resolution=0s&g0.deduplicate=1&g0.partial_response=0&g0.store_matches=%5B%5D

up:

== 20220114105525 AddIndexOnProjectsPath: migrating ===========================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:projects, :path, {:name=>"index_on_projects_path", :algorithm=>:concurrently})
   -> 0.0131s
-- add_index(:projects, :path, {:name=>"index_on_projects_path", :algorithm=>:concurrently})
   -> 0.0018s
== 20220114105525 AddIndexOnProjectsPath: migrated (0.0160s) ==================

down:

== 20220114105525 AddIndexOnProjectsPath: reverting ===========================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:projects, :path, {:name=>"index_on_projects_path", :algorithm=>:concurrently})
   -> 0.0566s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- remove_index(:projects, {:name=>"index_on_projects_path", :algorithm=>:concurrently, :column=>:path})
   -> 0.0195s
-- execute("RESET statement_timeout")
   -> 0.0005s
== 20220114105525 AddIndexOnProjectsPath: reverted (0.0810s) ==================

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Related to #349549 (closed)