Store the last time a repository was updated

What does this MR do?

The Geo Backfill only works the first time when the repository doesn't exist. We need backfill to be smarter and resync repositories that are broken or have not been updated in some time. To make this possible we can look at the timestamp of when the repository was last updated. This MR adds the changes to store the last time a repository was updated.

Are there points in the code the reviewer needs to double check?

Are there other points that we should update the last_repository_updated_at?

Why was this MR needed?

We can use events table to determine last push event but this will not be performant for large GitLab instances.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#2274 (closed)

/cc @stanhu