State the default state of a feature flag in GraphQL docs

What does this MR do?

In #335815 (closed) we changed the FF checking code in GraphQL to use the YAML definition default. This MR changes the code that generates the documentation in bundle exec rake gitlab:graphql:compile_docs to include the current default state of a feature flag.

How to test

1. Change the default_enabled: value in a feature flag definition file, e.g. :runner_graphql_query
2. Run bundle exec rake gitlab:graphql:compile_docs
3. The documentation should be updated to reflect the new value:

diff --git a/config/feature_flags/development/runner_graphql_query.yml b/config/feature_flags/development/runner_graphql_query.yml
index 6d6e7425a51..b7af0a2bb22 100644
--- a/config/feature_flags/development/runner_graphql_query.yml
+++ b/config/feature_flags/development/runner_graphql_query.yml
@@ -4,5 +4,5 @@ introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/59763
 rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/328700
 type: development
 group: group::runner
-default_enabled: false
+default_enabled: true
 milestone: '13.12'
diff --git a/doc/api/graphql/reference/index.md b/doc/api/graphql/reference/index.md
index 34c8fe5939b..cfb5acb4104 100644
--- a/doc/api/graphql/reference/index.md
+++ b/doc/api/graphql/reference/index.md
@@ -295,7 +295,7 @@ Returns [`QueryComplexity`](#querycomplexity).
 ### `Query.runner`
 
 Find a runner. Available only when feature flag `runner_graphql_query` is enabled.
-This flag is disabled by default, because the feature is experimental and is subject to change without notice.
+This flag is enabled by default.
 
 Returns [`CiRunner`](#cirunner).
 

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are 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.
• This change is backwards compatible across updates, or this does not apply.

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

Closes #335873 (closed)