Change the default concurrency on merge train

This commit changes the default concurrency on merge train

changelogs/unreleased/change-default-factor-on-merge-train.yml

+---
+title: Change the default concurrency factor of merge train to 20
+merge_request: 20201
+author:
+type: changed

doc/ci/merge_request_pipelines/pipelines_for_merged_results/merge_trains/index.md

@@ -32,8 +32,8 @@ Merge trains have the following requirements and limitations:
 - This feature requires that
   [pipelines for merged results](../index.md#pipelines-for-merged-results-premium) are
   **configured properly**.
-- Each merge train can run a maximum of **four** pipelines in parallel.
-  If more than four merge requests are added to the merge train, the merge requests
+- Each merge train can run a maximum of **twenty** pipelines in parallel.
+  If more than twenty merge requests are added to the merge train, the merge requests
   will be queued until a slot in the merge train is free. There is no limit to the
   number of merge requests that can be queued.
 - This feature does not support [squash and merge](../../../../user/project/merge_requests/squash_and_merge.md).

ee/app/services/merge_trains/refresh_merge_requests_service.rb

@@ -70,7 +70,7 @@ module MergeTrains
     def max_concurrency
       strong_memoize(:max_concurrency) do
         if Feature.enabled?(:merge_trains_parallel_pipelines, project, default_enabled: true)
-          if Feature.enabled?(:merge_trains_high_concurrency, project)
+          if Feature.enabled?(:merge_trains_high_concurrency, project, default_enabled: true)
             HIGH_CONCURRENCY
           elsif Feature.enabled?(:merge_trains_medium_concurrency, project)
             MEDIUM_CONCURRENCY