Derive WikiPage#last_version from existing data

What does this MR do?

We query the Gitaly WikiGetPageVersions RPC in a haml partial, which creates an N+1 problem #33951 (comment 232631025).

This MR instead derives the same "last version" Commit using the data that is already loaded, avoiding the need to call Gitaly here.

#34836

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• 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.
• [-] Tested in all supported browsers