Unpin docker version in gitlab-ci.yml

[jamesrkiger]

Checklist

1. If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. Ensure the tests pass
4. Make sure that your code lints and that you've followed our coding style
5. Write a title and, if necessary, a description of your work suitable for publishing in our release notes
6. Mention any related issues in this repository (as #ISSUE) and in other repositories (as kobotoolbox/other#ISSUE)
7. Open an issue in the docs if there are UI/UX changes

Description

The issue that required us to pin the docker version for the GitLab ci pipeline has been resolved. This PR unpins the version now that that short-term fix is no longer needed.

Related issues

Removes short-term fix from:
#911