Add severity mapping and engagement/mitigation SLA

Why is this change being made?

Customer's are asking for clarification on remediation timelines when it comes to vulnerabilities here at GitLab.

By updating remediation SLAs, include mitigation SLAs, and P1/S1 engagement clarifications we feel this new SLA table provides a more clear and concise document.

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