Reduce nested #capture_git_error calls on Wiki

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA

What does this MR do and why?

This MR refactors the wiki model to make #capture_git_error private in order to solve the problem raised in the issue #468185 (closed):

As far as I could see in the code base, I still think that we can make the #capture_git_error private, as it being used outside of the WikiPage seems like a leak of internals IMHO. This should be quite straightforward because:

• Currently, WikiPages::CreateService calls page.create from within a capture_git_error, but the page.create already uses the capture_git_error internally, so I don't see a reason why we're using it twice.
• Same thing happens on WikiPages::UpdateService, it's calling page.update from within a capture_git_error, but the page.update already calls the capture_git_error internally, therefore I think we can remove the external call as well.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.