Rails5 fix arel from

What does this MR do?

Fixes arel 7 from usage. The function was changed in https://github.com/rails/arel/commit/98fc25991137ee09b6800578117f8c1c322680f2. With the rails5 changes the count produces the exact same sql.

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

The same fix is probably needed in mysql_median_datetime_sql but I can't test that on Rails5

Why was this MR needed?

rails5 upgrade

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
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #46281 (closed)