Fix backward keyset pagination for merged_at

What does this MR do?

This MR fixes backward keyset pagination when ordering by merged_at.

In the Connection class we call last(N) (here) on the scope which reverses the order under the hood (relevant code).

When the ORDER BY expression is too complex, ActiveRecord cannot reverse it, so it raises ActiveRecord::IrreversibleOrderError.

This MR allows developers to "catch/detect" such queries and provide a custom order reverse strategy so the error is avoided.

The root cause is the NULLS LAST ordering, snippet example:

MergeRequest.order_merged_at(:asc).reverse_order

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team