Update deprecation note of env var in 2.2 release notes

The env var described in Deprecation Noticies won't be deprecated.

The request is to remove this section from the Deprecation Noticies.

UPDATE: More information on the changes that are required:

A) In the APIcast Environment Variables doc:

1. Add the documentation for the following parameters using the content from upstream (linked):

• APICAST_PROXY_HTTPS_CERTIFICATE
• APICAST_PROXY_HTTPS_CERTIFICATE_KEY
• APICAST_PROXY_HTTPS_SESSION_REUSE
• APICAST_PROXY_HTTPS_PASSWORD_FILE

2. Document APICAST_SERVICE_${ID}_CONFIGURATION_VERSION variable with the content from upstream.

3. Remove APICAST_REPORTING_THREADS, as it's an experimental feature and not supported yet.

B) In the Release Notes:

4. Remove all these environment variables from the "deprecated" list:

• APICAST_SERVICE_${ID}_CONFIGURATION_VERSION,
• APICAST_POLICY_LOAD_PATH,
• APICAST_PROXY_HTTPS_CERTIFICATE,
• APICAST_PROXY_HTTPS_CERTIFICATE_KEY,
• APICAST_PROXY_HTTPS_SESSION_REUSE and
• APICAST_PROXY_HTTPS_PASSWORD_FILE

5. Add the following environment variables to the "deprecated" list:

• APICAST_BACKEND_CACHE_HANDLER: Use caching policy instead. (+ add link to caching policy doc)
• APICAST_CUSTOM_CONFIG: Use policies instead. (+ add link to policies doc)
• APICAST_MODULE: Use policies instead. (+ add link to policies doc)