Allow users to authenticate via OAuth with password-based providers

What does this MR do and why?

Related to #429484 (closed)

Allows users to authenticate via OAuth with password-based providers. The fix is for an issue caused by a recent change to allow users to authenticate with LDAP and Crowd (password-based authentication) through the OAuth API that was added to prevent password spraying against this endpoint and locking out users. That however also impact users that relied on LDAP credentials to authenticate with this API. The fix has been merged into 16.7 but the bug has existed since 16.5 and we'd like to patch this for 16.6 so SM users aren't blocked until 16.7 is released

MR acceptance checklist

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

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