Disable db migrations on redis replicas in docs

What does this MR do and why?

Disable db migrations on redis replicas in docs

- When installing bundled redis using Omnibus GitLab, the directions
  for the replica redis nodes should include the same directive as the
  primary and disable database migrations.

Changelog: other

Signed-off-by: Robert Marshall <rmarshall@gitlab.com>

How to set up and validate locally

Follow the given directions for setting up a redis with replicas.

At reconfigure, the postgres migrations job will fail.

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.