Rails5 fix arel from in mysql_median_datetime_sql

What does this MR do?

Fix arel from usage in mysql_median_datetime_sql for rails 5. It's the same as !19340 (merged) but for mysql. Example spec that is now fixed: spec/models/cycle_analytics/review_spec.rb:28 failed with:

 1) CycleAnalytics#review when a regular merge request (that doesn't close the issue) is created and merged returns nil
     Failure/Error:
       query = arel_table
               .from(arel_table.project(Arel.sql('*')).order(arel_table[column_sym]).as(arel_table.table_name))
               .project(average([arel_table[column_sym]], 'median'))
               .where(
                 Arel::Nodes::Between.new(
                   Arel.sql("(select @row_id := @row_id + 1)"),
                   Arel::Nodes::And.new(
                     [Arel.sql('@ct/2.0'),
                      Arel.sql('@ct/2.0 + 1')]
                   )

     ArgumentError:
       wrong number of arguments (given 1, expected 0)

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

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)

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