Ignore storage_version column in snippet table
What does this MR do?
In !22275 (merged), we added the columns repository_storage and storage_version to the snippet table. Nevertheless, after #39176 (comment 270821384) it seems we don't need storage_version since it's a column needed only for legacy storage and we're only using hashed storage in snippet repositories.
According to https://docs.gitlab.com/ee/development/what_requires_downtime.html#dropping-columns, the first step is to ignore_column the column.