After all PR close: Verify machineKey decryptionKey have been remediated

[Rick-Anderson]

Once #34365 is closed:

• Validate all locals of machineKey decryptionKey have been remediated
• Do a robust search to determine no literal keys remain.
  use this regex for the search:
  \b(validation|decryption)Key\s*=\s*\"[A-Fa-f0-9]{2}

This should eliminate false positives like <machineKey decryptionKey="AutoGenerate,IsolateApps" />. Such false positives seem to be fairly prevalent and would pollute a simple <machineKey substring search.