Backport of update alias lookahead to respect case into release/1.20.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #31352 to be assessed for backporting due to the inclusion of the label backport/1.20.x.

The below text is copied from the body of the original PR.

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Description

update alias lookahead to respect case:

• userpass is not case sensitive
• ldap is case sensitive when it is configured that way

TODO only if you're a HashiCorp employee

• Backport Labels: If this fix needs to be backported, use the appropriate backport/ label that matches the desired release branch. Note that in the CE repo, the latest release branch will look like backport/x.x.x, but older release branches will be backport/ent/x.x.x+ent.
  • LTS: If this fixes a critical security vulnerability or severity 1 bug, it will also need to be backported to the current LTS versions of Vault. To ensure this, use all available enterprise labels.
• ENT PR: If this change has an associated ENT PR, please link it in the
  description. Also, make sure the changelog is in this PR, not in your ENT PR.

PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.

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Overview of commits

• 881febb

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CI Results:
All Go tests succeeded! ✅

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Build Results:
All builds succeeded! ✅