changelog: add fleet ingest pipeline breaking change (elastic#12556)

* changelog: add fleet ingest pipeline breaking change

See elastic/kibana#175254

Part of elastic#12498

Author: carsonip

changelogs/8.12.asciidoc

@@ -13,6 +13,7 @@ https://github.com/elastic/apm-server/compare/v8.11.4\...v8.12.0[View commits]
 ==== Breaking Changes
 - The unsupported apm_data_stream_migration ingest pipeline has been removed {pull}12102[12102].
 - "publish_ready" is always false in the "GET /" response until events are received by apm-server {pull}12150[12150]
+- A bug has been introduced, so that `traces-apm@custom` ingest pipeline is now additionally applied to data streams `traces-apm.sampled-*` and `traces-apm.rum-*` and applied twice for `traces-apm-*`. See more details and information around a fix in the Known Issues section.
 
 [float]
 ==== Bug fixes