Fix external Redis reconciliation and associated tests

What does this MR do?

Change 'redis.password' key to 'redis.auth'

Changes the 'redis.password' configuration key to 'redis.auth' to align with the associated change in the GitLab Charts: gitlab-org/charts/gitlab@6e07c473

Changelog: changed

Note: tests were already fixed in f4a83b38.

Note: includes fix from !789 (merged).

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added (tests already existed, we just needed to change the key that's checked)
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.

Related issues

Related to #1561 (comment 1858988548)

Test plan

✅ Confirm that the slow_unit_tests jobs pass.