Fix rubocop todos for ClusterReindexingService & spec

What does this MR do and why?

Fix some rubocop todos for Elastic::ClusterReinexingService and the spec

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

N/A

Before	After

How to set up and validate locally

Code organization/format changes only