build: release 6.100.0

[tshuli]

Problem

When we rolled out encryption boundary shift and virus scanner, we built in a fallback mechanism to the encrypt submission mode in case of error.

This PR:

• Drops the frontend fallback to encrypt submission mode
• Removes the encrypt submission route on the backend
• Removes the feature flags

Tests

Regresssion

• Storage mode form with multiple attachment works
• Block POST network calls to https://s3.ap-southeast-1.amazonaws.com. The retry should still try to upload the file to https://s3.ap-southeast-1.amazonaws.com (as opposed to submitting via encrypt submission route)
• Email mode form with multiple attachment works

On GSIB

• Storage mode form with multiple attachment works
• Email mode form with multiple attachment works

Deploy notes

• After release is stable, remove the following feature flags from DB:
  • encryptionBoundaryShift: 'encryption-boundary-shift'
  • encryptionBoundaryShiftHardValidation: 'encryption-boundary-shift-hard-validation'
  • encryptionBoundaryShiftVirusScanner: 'encryption-boundary-shift-virus-scanner'

[KenLSM]

LGTM!