Expanding RM Elevated permissions steps to cover gitaly
Release Manager "Direct push permissions" have been removed from the Gitaly master branch to comply with https://gitlab.com/gitlab-com/gl-security/security-assurance/security-compliance-commercial-and-dedicated/observation-management/-/issues/876#note_1509010821
This MR expands the EM guide to granting these permissions if needed in times of need.