Remove NOT NULL on policy_id for scan_result_policy_violations

What does this MR do and why?

This change updates scan_result_policy_violations table to remove the NOT NULL constraint on scan_result_policy_id as it will be replaced by approval_policy_rule_id as a part of Use database read model for merge request appr... (&9971)

Migration Output

Migrate Up

main: == [advisory_lock_connection] object_id: 128500, pg_backend_pid: 15855
main: == 20240704123140 RemoveScanResultPolicyViolationsPolicyIdNotNull: migrating ==
main: -- execute("ALTER TABLE scan_result_policy_violations ALTER COLUMN scan_result_policy_id DROP NOT NULL")
main:    -> 0.0022s
main: == 20240704123140 RemoveScanResultPolicyViolationsPolicyIdNotNull: migrated (0.0092s) 

main: == [advisory_lock_connection] object_id: 128500, pg_backend_pid: 15855

main: == [advisory_lock_connection] object_id: 133120, pg_backend_pid: 36853
main: == 20240709115642 AddApprovalPolicyIdConstraintToScanResultPolicyViolations: migrating 
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- execute("ALTER TABLE scan_result_policy_violations\nADD CONSTRAINT chk_policy_violations_rule_id_or_policy_id_not_null\nCHECK ( approval_policy_rule_id IS NOT NULL OR scan_result_policy_id IS NOT NULL )\nNOT VALID;\n")
main:    -> 0.0020s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0010s
main: -- execute("ALTER TABLE scan_result_policy_violations VALIDATE CONSTRAINT chk_policy_violations_rule_id_or_policy_id_not_null;")
main:    -> 0.0045s
main: -- execute("RESET statement_timeout")
main:    -> 0.0016s
main: == 20240709115642 AddApprovalPolicyIdConstraintToScanResultPolicyViolations: migrated (0.1002s) 
main: == [advisory_lock_connection] object_id: 133120, pg_backend_pid: 36853

Migrate Down

main: == [advisory_lock_connection] object_id: 128500, pg_backend_pid: 15386
main: == 20240704123140 RemoveScanResultPolicyViolationsPolicyIdNotNull: reverting ==
main: -- execute("ALTER TABLE scan_result_policy_violations ALTER COLUMN scan_result_policy_id SET NOT NULL")
main:    -> 0.0025s
main: == 20240704123140 RemoveScanResultPolicyViolationsPolicyIdNotNull: reverted (0.0073s) 

main: == [advisory_lock_connection] object_id: 128500, pg_backend_pid: 15386

MR acceptance checklist

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Addresses #464034