Enable codequality_mr_diff feature flag by default

What does this MR do?

For #284140 (closed) / #267612 (closed)

This MR enables the codequality_mr_diff feature flag by default. The flag has been enabled for the GitLab project/group since 2021-05-07, and enabled globally since 2021-05-14, and we have not had any issues so far 🤞🏻

Technically, this MR

• sets the feature flag codequality_mr_diff to default_enabled: true in the .yml file where it is defined
• changes the value of default_enabled in the Feature.enabled? check to :yaml so that it depends on the value from the .yml file
• adds a changelog entry for the feature that we're enabling by default
• adds a line to the docs about the feature flag's new default state and changes the instructions below from how to enable the flag to how to disable the flag

Screenshots

There should be no visual changes, but here's the badge anyway:

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?) - this is an EE feature, so the changelog is in ee/
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?) - no database changes
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines) - we have not encountered any performance problems so far during the feature flag rollout
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.) - no test changes needed at this point
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed. - not needed