Backport of 'Map sast_iac scanner to sast in approval policy rules'

What does this MR do and why?

Backports !267407 (merged) to 19-4-stable-ee.

Approval policies that list sast_iac under a scan_finding rule's scanners never get an approval rule. The policy schema accepts the value, but ApprovalProjectRule only accepts security report types, so the whole scanners list fails validation and no approval rule is created. The failure is only logged at debug level, so the policy silently does nothing. The fix maps sast_iac to sast (the report type IaC scanning uses) when the rule is read, and removes duplicate scanners.

Original issue: #637971 (severity 2, bug::functional).

Why backport: the bug leaves affected approval policies unenforced with no visible error, and there is no workaround short of editing the policy. The change is small (Reviewable LOC 14 plus specs), has no feature flag and no migrations.

Cherry-pick: git cherry-pick -x -m1 63301495fdcb412c190dc43be452993ac2eb5679. One trivial conflict in ee/lib/security/scan_result_policies/rule.rb: on master the new normalize_scanner_type method sits next to dependency_filter, which does not exist on this branch. I kept only normalize_scanner_type. The rest of the diff applied as-is.

Policies that were already affected need resyncSecurityPolicies after the fix is deployed.

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