Set improved_mr_merged_at_queries feature flag to enabled by default
Problem to solve
The improved_mr_merged_at_queries
feature flag was introduced in !39329 (merged). In !40138 (comment 401445411), @ahegyi noted "I think we can enable it by default now, since it's working as expected."
Proposal
The improved_mr_merged_at_queries
feature flag should be enabled by default.