[Rails5] Fix MysqlDateTimeWithTimeZone in rails 5

What does this MR do?

Mysql fixes for Rails 5

• MysqlDateTimeWithTimeZone inherits from ActiveRecord::Type::DateTime (MysqlDateTime is not present in Rails 5)
• explicitly set NULL default value for merge_request_diff_files's diff column (otherwise empty string is used in a migration) and empty string is not allowed for text/blob fields in Mysql
• disable NO_ZERO_DATE mode for all Mysql DB connections, otherwise SQL queries fail on inserting 0 value for created_at column. To make sure that configure_connection patch takes effect, this patch is loaded right in the config/application.rb before Rails connects to DB.

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
• 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 #46275 (closed)