Use find_or_create_by to avoid subtransactions

Use `find_or_create_by` instead of `safe_find_or_create_by` to avoid
subtransactions.

db/post_migrate/20210901184511_prepare_async_indexes_for_ci_builds.rb

@@ -67,7 +67,7 @@ class PrepareAsyncIndexesForCiBuilds < Gitlab::Database::Migration[1.0]
 
     return if index_name_exists?(table_name, index_name)
 
-    async_index = Gitlab::Database::AsyncIndexes::PostgresAsyncIndex.safe_find_or_create_by!(name: index_name) do |rec|
+    async_index = Gitlab::Database::AsyncIndexes::PostgresAsyncIndex.find_or_create_by!(name: index_name) do |rec|
       rec.table_name = table_name
       rec.definition = definition
     end