Add a note about when to apply hot patches

What does this MR do?

Expands the merge request body with a note specifying when it's appropriate to proceed with hot-patching

As an action item of gitlab-org/release/retrospectives#35 (closed)

Example of a merge request

@john-doe has initiated a hot patch for incident https://gitlab.com/gitlab-com/gl-infra/production/-/issues/2520

NOTE: Hot patch should only proceed if GitLab.com is down or if there's evidence of an S1 security vulnerability being actively exploited, for all other scenarios please rely on the auto-deploy process.

Please see the release-docs for instructions

TODO

• Disable auto-deployments by setting CNY_MANUAL_PROMOTE as a CI variable in https://ops.gitlab.net/gitlab-com/gl-infra/deployer
• Ensure a developer is working on a fix, and the MR has the appropriate labels (e.g. Pick into auto-deploy)

cc @gitlab-org/release/managers