Add check_type enum to PreScanVerificationStep

What does this MR do and why?

As suggested here we created a follow-up issue to change the DastPreScanVerification attribute name to an enum.

And since the Changing column type process didn't work as described here, we opted to create a new column (Internal only).

This MR also starts the drop column process for the old column.

Migration

up:

main: == 20230217144421 AddCheckTypeToPreScanStep: migrating ========================
main: -- add_column(:dast_pre_scan_verification_steps, :check_type, :integer, {:limit=>2, :default=>0, :null=>false})
main:    -> 0.0042s
main: == 20230217144421 AddCheckTypeToPreScanStep: migrated (0.0079s) ===============

down:

main: == 20230217144421 AddCheckTypeToPreScanStep: reverting ========================
main: -- remove_column(:dast_pre_scan_verification_steps, :check_type)
main:    -> 0.0028s
main: == 20230217144421 AddCheckTypeToPreScanStep: reverted (0.0136s) ===============

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