Bump redis-namespace to 1.9.0

This MR aims to:

• formalize a test around https://gitlab.com/gitlab-org/gitlab-exporter/-/issues/92+
• fix the behavior by bumping redis-namespace

When using Ruby 3 the following spec:

• fails using redis-namespace 1.6.0
• passes using redis-namespace 1.9.0

module GitLab
  module Exporter
    module CLI
      describe SidekiqRunner, :integration do
        let(:redis_url) { ENV.fetch("REDIS_URL", "redis://localhost:6379") }
        let(:io) { StringIO.new }

        it "can properly reach out to redis" do
          args = CLIArgs.new([io], options: { /redis-url/ => redis_url })
          runner = SidekiqRunner.new(args)

          expect { runner.run }.not_to raise_error
        end
      end
    end
  end
end