Add Terraform module scan limitation note to IaC SAST docs

What does this MR do?

The IaC SAST scanner currently does not support scanning for vulnerabilities in custom Terraform modules from e.g. the GitLab registry. We've discussed the missing feature in &6653 (comment 847160211) with suggesting to add a note to the docs. This MR adds a note into the Terraform table, documenting and linking the GitLab feature proposal in IaC Scanning should detect vulnerabilities in c... (#357004)

Related issues

• #357004

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