Merge branch 'update-shell' into '8-12-stable'

Bump Gitlab Shell to support low IO priority for storage moves

## What does this MR do?

Bump Gitlab Shell to support low IO priority for storage moves

## Are there points in the code the reviewer needs to double check?

No

## Why was this MR needed?

We need this shell enhancement to continue to move projects in production 

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

https://gitlab.com/gitlab-com/infrastructure/issues/443

See merge request !6525

CHANGELOG

@@ -16,6 +16,7 @@ v 8.12.2
   - Only update issuable labels if they have been changed
   - Fix bug where 'Search results' repeated many times when a search in the emoji search form is cleared (Xavier Bick) (@zeiv)
   - Fix resolve discussion buttons endpoint path
+  - Update Gitlab Shell to support low IO priority for storage moves
 
 v 8.12.1
   - Fix a memory leak in HTML::Pipeline::SanitizationFilter::WHITELIST

GITLAB_SHELL_VERSION

-3.6.0
+3.6.1