Update Oauth token hashing strategies

What does this MR do and why?

The changes modify how OAuth tokens and secrets are stored and validated, including FIPS mode support and fallback strategies. SHA512 is now used for application secrets instead of PBKDF2, and fallback strategies are adjusted for FIPS compliance.

This MR

• Swap application secret hashing mechanism to SHA512
• Remove SHA512 as the fallback mechanism which was added as the temporary transition mechanism
• Skip PBKDF2 fallback when FIPS mode enabled.

References

• Fixes #551169 (closed) and #548736 (closed)

• Revert MR due the bug in fallback mechanism: !202215 (merged)

• MR which fixes fallback: !202322 (merged)

• Incident: https://app.incident.io/gitlab/incidents/3403

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