Bump up @gitlab/eslint-config dependency version to 1.6.0

What does this MR do?

Bumps up the version of the @gitlab/eslint-config dependency from 1.5.0 to 1.6.0 to begin enforcing the no-implicit-coercion ESLint rule implemented in https://gitlab.com/gitlab-org/gitlab-eslint-config/merge_requests/14.

The fixes for this rule have already been made in CE and EE:

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29007
• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13803