Remove storage_version column from snippets

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.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content