Update SG.5.03 control statement

Why is this change being made?

This MR updates the control statement for SG.5.03. The original control statement called out that roles and responsibilities were documented within the GitLab intranet. Given that we don't have an intranet, the control statement has been updated to correctly state that this information is documented in the GitLab handbook.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what