Soft cutoff date of the 15th for engineering MRs

Problem to solve

@wortschi pointed out that we have recurring trouble with our Release Posts being included when they shouldn't, or not included when they should. They are also frequently rushed at release time, which is already our busiest time.

Part of the problem is that the Release Post instruction say: "by the 17th EMs merge those MRs in to master as the features they represent are merged in to the GitLab codebase." However, the Release instructions say: "Merging by the 17th does not guarantee that the feature will be in the [milestone] release. See release timelines."

Proposal

Consider the 15th a soft cutoff date for engineering MRs to be merged for a release.