Update redis-namespace to 1.6.0

What does this MR do?

Updates redis-namespace from 1.5.2 to 1.6.0.

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

• Upstream diff: https://github.com/resque/redis-namespace/compare/v1.5.2...v1.6.0

Why was this MR needed?

To get officially supported for Ruby 2.3/2.4.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests passed for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides

What are the relevant issue numbers?

Closes #46846 (closed)