Forbid snippet pushes when repo is read-only

What does this MR do?

In the epic &3393 (closed) we implemented all the logic to move snippet repositories from one shard to another. Nevertheless, we forgot to forbid repository pushes when the snippet repository is read-only (we can allow read access to the repo).

In this MR we implement that logic.

Does this MR meet the acceptance criteria?

Conformity

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