Enables the mr_changes_fluid_layout flag by default

This flag makes it so that diffs will use our fluid layout
container instead of limited even if the user does not
have that option selected as a preference.

Changelog: added

config/feature_flags/development/mr_changes_fluid_layout.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/341809
 milestone: '14.4'
 type: development
 group: group::code review
-default_enabled: false
+default_enabled: true