Remove lock version indexes

Removes the following indexes if they exist.

- index_issues_on_lock_version
- index_merge_requests_on_lock_version
- index_epics_on_lock_version

Changelog: fixed

Relates to https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77674

db/migrate/20220106141756_remove_lock_version_indexes.rb

+# frozen_string_literal: true
+
+class RemoveLockVersionIndexes < Gitlab::Database::Migration[1.0]
+  disable_ddl_transaction!
+
+  INDEXES = {
+    issues: 'index_issues_on_lock_version',
+    merge_requests: 'index_merge_requests_on_lock_version',
+    epics: 'index_epics_on_lock_version'
+  }
+
+  def up
+    INDEXES.each do |table, index_name|
+      remove_concurrent_index_by_name table, index_name
+    end
+  end
+
+  def down
+    INDEXES.each do |table, index_name|
+      add_concurrent_index table, :lock_version, where: "lock_version IS NULL", name: index_name
+    end
+  end
+end

db/schema_migrations/20220106141756

+cedca81a6dc1562cf0ed5f17217c52420a1cb9a74acece8d67eb1bcb948a7181
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