Enable API v3 `branches/:sha` partial response

https://gitlab.com/gitlab-org/gitlab/-/issues/344617
https://gitlab.com/gitlab-org/gitlab/-/issues/331326

Enabling the feature flag by default to enable protecting API v3
`commits/:sha` endpoint from causing frequent Gitaly timeouts.

It does this by returning a partial response without the commit diffs.

Changelog: performance

config/feature_flags/development/api_v3_commits_skip_diff_files.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/344617
 milestone: '14.5'
 type: development
 group: group::integrations
-default_enabled: false
+default_enabled: true

lib/api/v3/github.rb

@@ -101,7 +101,7 @@ module API
         # of time after a Gitaly timeout, to mitigate frequent Gitaly timeouts
         # for some Commit diffs.
         def diff_files(commit)
-          return commit.diffs.diff_files unless Feature.enabled?(:api_v3_commits_skip_diff_files, commit.project)
+          return commit.diffs.diff_files unless Feature.enabled?(:api_v3_commits_skip_diff_files, commit.project, default_enabled: :yaml)
 
           cache_key = [
             GITALY_TIMEOUT_CACHE_KEY,