Add changelog for enabling feature flag

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

changelogs/unreleased/default-enabled-api_always_use_application_json.yml

+---
+title: Change default API content_type to JSON
+merge_request: 54479
+author:
+type: changed

config/feature_flags/development/api_always_use_application_json.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/270067
 milestone: '13.6'
 type: development
 group: group::ecosystem
-default_enabled: false
+default_enabled: true

doc/api/README.md

@@ -750,7 +750,7 @@ specifically used by GitLab.com, see
 The GitLab API supports the `application/json` content type by default, though
 some API endpoints also support `text/plain`.
 
-In [GitLab 13.9 and later](https://gitlab.com/gitlab-org/gitlab/-/issues/250342),
+In [GitLab 13.10 and later](https://gitlab.com/gitlab-org/gitlab/-/issues/250342),
 API endpoints do not support `text/plain` by default, unless it's explicitly documented.
 
 This change is deployed behind the `:api_always_use_application_json` [feature flag](../user/feature_flags.md),

lib/api/api.rb

@@ -147,7 +147,7 @@ module API
 
       # Only overwrite `text/plain+deprecated`
       if content_types[api_format] == 'text/plain+deprecated'
-        if Feature.enabled?(:api_always_use_application_json)
+        if Feature.enabled?(:api_always_use_application_json, default_enabled: :yaml)
           content_type 'application/json'
         else
           content_type 'text/plain'