WIP: Fix Ci::Runner#contacted_at is sometimes a string which breaks runner admin page

What does this MR do?

At the moment the runner admin page is broken and will not load. spec/features/admin/admin_runners_spec.rb is also broken but does not seem to fail in CI so I'm not 100% sure. This should fix the problem:

 ActionView::Template::Error:
       Object must be a Date, DateTime or Time object. "2018-05-04T07:29:50.365+02:00" given.
     # ./app/helpers/application_helper.rb:107:in `time_ago_with_tooltip'
     # ./app/views/admin/runners/_runner.html.haml:34:in `_app_views_admin_runners__runner_html_haml___349973184012124948_70097738702620'
     # ./app/views/admin/runners/index.html.haml:71:in `block in _app_views_admin_runners_index_html_haml__3432066315448256266_70097737665360'
     # ./app/views/admin/runners/index.html.haml:70:in `_app_views_admin_runners_index_html_haml__3432066315448256266_70097737665360'

I believe this regression was introduced in ebcd5711

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

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #46082 (closed)