Update to Incident issue template to support FCLs and IMOC expansion
Teams participating in FCLs are asked to publish an RCA in the incident issue. https://about.gitlab.com/handbook/engineering/#feature-change-locks
This MR clarifies the RCA related sections of the incident issue template and brings it inline with other widely used RCA templates such as https://about.gitlab.com/handbook/engineering/root-cause-analysis/ and https://gitlab.com/gitlab-com/gl-security/security-operations/sirt/operations/-/blob/master/.gitlab/issue_templates/rca.md
A preview of the updated template is available in: gitlab-com/www-gitlab-com#12454