Enable optimized merged at count FF by default

What does this MR do?

This MR enables the optimized_merge_request_count_with_merged_at_filter feature flag by default (introduced by this MR). The flag was enabled on PRD a week ago and so far no errors were reported.

The performance improvement of the MR analytics chart has been verified: #276386 (comment 495096013)

Note: the changelog is for EE because the chart which is using the new GraphQL API changes is in EE.

How to test it

The chart should load under 2s:

https://gitlab.com/gitlab-org/gitlab/-/analytics/merge_request_analytics?start_date=2020-02-10&end_date=2021-02-09

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

Related to #299347 (closed)