Expire snippet repo size caching

What does this MR do?

Ref: #213526 (closed)

Expire snippet repository size caching after push.

Since this is still behind feature flag no changelog is required.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.