Recommended Unicorn memory limit changes in 12.7

This change reflects a small but important note in our unicorn-worker-killer doc: https://docs.gitlab.com/ee/administration/operations/unicorn.html#unicorn-worker-killer

The "changes in GitLab 12" page is a key source of information for customers, TAMs, and support during upgrades, and so this is an important change. The value here is changed in the gitlab.rb file, which is not changed as part of the upgrade process so most customers will not realize this change is needed until they start seeing failures. Including it in the version 12 changes page can help mitigate those kinds of issues.