Add in migration guide that env var APICAST_ENVIRONMENT will be removed and should be set in APImanager CRD

Configuring Custom logging policy globally until 2.10 is done by mounting an configmap and using the env var APICAST_ENVIRONMENT. When upgrading from 2.10 to 2.11 the Operator reconciles the env var and is missing in 2.11.

In 2.11 the env var APICAST_ENVIRONMENT must be configured as a secret and referenced in the APImanager CRD as explained here..

This represents a breaking change when upgrading from 2.10 to 2.11 when using the operator.

Configuring global policy globally is only documented in 2.11 with a configmap for template based deployment in OCP 3.11.

Notes for Docs

From this comment in the MR:

I have some comments:

I think the title "4.17.1. Adding custom environments for the 3scale operator" doesn't apply for the section, and would be confusing for the customers. This env var is used to configure the logging policy in this specific use case and it's the only possible solution.
Because of 1) and also because there is a difference on how to configure the logging policy globally with template and operator I'd split the section: Global configuration for all APIs in 2 subsections:

Global configuration for all APIs > Template based deployment
Global configuration for all APIs > Operator based deployment

The introduction and example of custom_env.lua applies in both cases so having the example once will suffice. The difference is how you mount the file. In template based the file is mounted with the docker command shown (from this part: "To run APIcast with this specific environment:") and with operator based with the secret.