Upgrade Workhorse to v8.38.0

This adds support for adding S3 encryption headers to uploads. Full list
of changes:
https://gitlab.com/gitlab-org/gitlab-workhorse/-/blob/master/CHANGELOG

GITLAB_WORKHORSE_VERSION

-8.37.0
+8.38.0

changelogs/unreleased/sh-bump-workhorse-8-38-0.yml

+---
+title: Upgrade Workhorse to v8.38.0
+merge_request: 39223
+author:
+type: added