Upgrade gitlab-elasticsearch-indexer to version 2.15.0

What does this MR do and why?

Bumps up the version of the gitlab-elasticsearch-indexer to 2.15.0

The changes include:

• Drop support for go version 1.15
• Update go version to 1.16
• Replace deprecated go package io/ioutil

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

GITLAB_ELASTICSEARCH_INDEXER_VERSION

-2.14.0
+2.15.0