Replace `.exists` with `EXISTS ()`

What does this MR do?

It replaces .exists call with more explicit EXIST () - the generated sql query should be same in both cases.

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

Why was this MR needed?

• .exists is not part of public AR's API, it should not be used
• there is a bug in early version of rails 5 which causes that limit() doesn't work in combination with exists - https://github.com/rails/rails/issues/16959

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 #46277 (closed)