Fix SQL: key is reserved word

Meant to catch this in the prior commit: `key` is a SQL reserved word
and therefore must have backticks around it.

MIGRATING.md

@@ -134,7 +134,7 @@ class ConvertToActiveStorage < ActiveRecord::Migration[5.2]
 
     active_storage_blob_statement = ActiveRecord::Base.connection.raw_connection.prepare('active_storage_blob_statement', <<-SQL)
       INSERT INTO active_storage_blobs (
-        key, filename, content_type, metadata, byte_size, checksum, created_at
+        `key`, filename, content_type, metadata, byte_size, checksum, created_at
       ) VALUES ($1, $2, $3, '{}', $4, $5, $6)
     SQL