helm: fix regression of #15243

[sathieu]

The previous behavior was to use accessKey as secret key.

Description

Motivation and Context

#15243 broke my install with an error.

How to test this PR?

  users:
  - accessKey: "foobar"
    existingSecret: minio-secretkeys-gitops
    existingSecretKey: "foobar"  # actually ignored
    policy: "readwrite_test"

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression Fix Helm chart does not support special characters in access/secret key #15243 ab9544c
• Documentation updated
• Unit tests added/updated

[minio-trusted]

Mint Automation

Test	Result
mint-large-bucket.sh	✔️
mint-fs.sh	✔️
mint-gateway-s3.sh	✔️
mint-erasure.sh	✔️
mint-dist-erasure.sh	✔️
mint-compress-encrypt-dist-erasure.sh	✔️
mint-pools.sh	✔️
Deleting image on docker hub
Deleting image locally