Use dedicated signing key for CI_JOB_JWT by default

This updates the `ci_jwt_signing_key` feature flag to be enabled by
default.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/258546.

changelogs/unreleased/ci-jwt-signing-key-enable-by-default.yml

+---
+title: Use dedicated signing key for CI_JOB_JWT by default
+merge_request: 47336
+author:
+type: changed

config/feature_flags/development/ci_jwt_signing_key.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/258546
 milestone: '13.6'
 type: development
 group: group::release management
-default_enabled: false
+default_enabled: true

doc/ci/examples/authenticating-with-hashicorp-vault/index.md

@@ -56,7 +56,7 @@ The JWT's payload looks like this:
 }
 ```
 
-The JWT is encoded by using RS256 and signed with your GitLab instance's OpenID Connect private key. The expire time for the token will be set to job's timeout, if specified, or 5 minutes if it is not. The key used to sign this token may change without any notice. In such case retrying the job will generate new JWT using the current signing key.
+The JWT is encoded by using RS256 and signed with a dedicated private key. The expire time for the token will be set to job's timeout, if specified, or 5 minutes if it is not. The key used to sign this token may change without any notice. In such case retrying the job will generate new JWT using the current signing key.
 
 You can use this JWT and your instance's JWKS endpoint (`https://gitlab.example.com/-/jwks`) to authenticate with a Vault server that is configured to allow the JWT Authentication method for authentication.
 

lib/gitlab/ci/jwt.rb

@@ -63,7 +63,7 @@ module Gitlab
 
       def key
         @key ||= begin
-                   key_data = if Feature.enabled?(:ci_jwt_signing_key, build.project)
+                   key_data = if Feature.enabled?(:ci_jwt_signing_key, build.project, default_enabled: true)
                                 Gitlab::CurrentSettings.ci_jwt_signing_key
                               else
                                 Rails.application.secrets.openid_connect_signing_key