Wiki repository size isn't updated after migration

What does this MR do and why?

As per #393547 (closed), we have to update the Wiki size, since:

For projects migrated via GitLab Direct Transfer, the Wiki report size reports 0 bytes, although the wiki was successfully migrated.

How to set up and validate locally

1. Checkout the bmarjanovic-393547-wiki-repository-size-isn-t-updated-after-migration branch
2. Import a project with Wiki using the GitLab Direct Transfer
3. Check the latest project's statistics
   • Project.last.statistics.wiki_size
4. Validate that the wiki_size is correctly updated

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #393547 (closed)