Document recent changes to nginx fileset as breaking changes (#18026) (…

…#18113)

PR #17844 introduced the following changes:

- http.request.method is now lowercase
- http.request.referrer is only set when nginx provides a value

This PR updates the CHANGELOG to report this as a breaking change.

(cherry picked from commit 0de9b67)

CHANGELOG.next.asciidoc

@@ -33,6 +33,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Fix parsing of Elasticsearch node name by `elasticsearch/slowlog` fileset. {pull}14547[14547]
 - CEF extensions are now mapped to the data types defined in the CEF guide. {pull}14342[14342]
 - Improve ECS field mappings in panw module.  event.outcome now only contains success/failure per ECS specification. {issue}16025[16025] {pull}17910[17910]
+- Improve ECS categorization field mappings for nginx module. http.request.referrer is now lowercase & http.request.referrer only populated when nginx sets a value {issue}16174[16174] {pull}17844[17844]
 
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