Enable epic_boards feature flag by default

What does this MR do?

Enables :epic_boards feature flag by default, aiming for 14.0 release.

History of Epic boards feature development

Previous iterations	MR link
Index page	!53100 (merged)
Empty lists	!54169 (merged)
Epics in lists	!54528 (merged)
Multiple epic boards switcher	!54399 (merged)
Create epic board	!54642 (merged)
Display epic reference in card	!55028 (merged)
Add label list	!55011 (merged)
Collapse list and polish	!55456 (merged)
MVC1 bug fixes	!55709 (merged)
Drag & Drop epic	!56170 (merged)
Fix mentions of issues to epics	!56292 (merged)
Update board scope	!55815 (merged)
Delete epic board	!57186 (merged)
Edit epic labels	!57973 (merged)
Edit epic title	!58727 (merged)
Edit epic confidentiality	!58968 (merged)
Edit epic subscribed state	!59214 (merged)
Edit epic start and due dates	!59446 (merged)
Delete epic board list	!60559 (merged)
Display epic's participants in sidebar	!60885 (merged)

Screenshots (strongly suggested)

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?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related 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)
• 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.)
• 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.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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 #290039 (closed)

ee/app/controllers/groups/epic_boards_controller.rb

@@ -71,6 +71,6 @@ def assign_endpoint_vars
   end
 
   def authorize_read_board!
-    access_denied! unless Feature.enabled?(:epic_boards, group) && can?(current_user, :read_epic_board, group)
+    access_denied! unless Feature.enabled?(:epic_boards, group, default_enabled: :yaml) && can?(current_user, :read_epic_board, group)
   end
 end