Add unicorn memory limit for 12.6 and older

doc/administration/operations/unicorn.md

@@ -64,13 +64,19 @@ between requests_, so no user requests are affected. You can set the minimum and
 maximum memory threshold (in bytes) for the Unicorn worker killer by
 setting the following values `/etc/gitlab/gitlab.rb`:
 
-```ruby
-unicorn['worker_memory_limit_min'] = "1024 * 1 << 20"
-unicorn['worker_memory_limit_max'] = "1280 * 1 << 20"
-```
-
-NOTE: **Note:**
-These values apply to GitLab 12.7.0 or newer versions. For older GitLab versions please consult [previous worker memory limits](https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/12.6.0+ee.0/files/gitlab-cookbooks/gitlab/attributes/default.rb#L422-423).
+- For GitLab **12.7** and newer:
+  
+  ```ruby
+  unicorn['worker_memory_limit_min'] = "1024 * 1 << 20"
+  unicorn['worker_memory_limit_max'] = "1280 * 1 << 20"
+  ```
+
+- For GitLab **12.6** and older:
+
+  ```ruby
+  unicorn['worker_memory_limit_min'] = "400 * 1 << 20"
+  unicorn['worker_memory_limit_max'] = "650 * 1 << 20"
+  ```
 
 Otherwise, you can set the `GITLAB_UNICORN_MEMORY_MIN` and `GITLAB_UNICORN_MEMORY_MAX`
 [environment variables](../environment_variables.md).