[DOCS] Migration information about ES logging breaking changes (#79146)

Adds breaking change docs for #47105. (ECS layout and plaintext log files removal)

Relates to #46119

Author: pgomulka

docs/reference/migration/migrate_8_0.asciidoc

@@ -24,6 +24,7 @@ coming[8.0.0]
 * <<breaking_80_indices_changes>>
 * <<breaking_80_ingest_changes>>
 * <<breaking_80_java_changes>>
+* <<breaking_80_logging_changes>>
 * <<breaking_80_mappings_changes>>
 * <<breaking_80_network_changes>>
 * <<breaking_80_node_changes>>
@@ -126,6 +127,7 @@ include::migrate_8_0/ilm.asciidoc[]
 include::migrate_8_0/indices.asciidoc[]
 include::migrate_8_0/ingest.asciidoc[]
 include::migrate_8_0/java.asciidoc[]
+include::migrate_8_0/logging.asciidoc[]
 include::migrate_8_0/mappings.asciidoc[]
 include::migrate_8_0/network.asciidoc[]
 include::migrate_8_0/node.asciidoc[]

docs/reference/migration/migrate_8_0/logging.asciidoc

@@ -0,0 +1,42 @@
+[discrete]
+[[breaking_80_logging_changes]]
+==== Logging changes
+
+//NOTE: The notable-breaking-changes tagged regions are re-used in the
+//Installation and Upgrade Guide
+
+//tag::notable-breaking-changes[]
+.{es} JSON logs now comply with ECS.
+[%collapsible]
+====
+*Details* +
+{es}'s {ref}/logging.html[JSON logs] now comply with the
+{ecs-ref}/index.html[Elastic Common Schema (ECS)]. Previously, {es}'s JSON logs
+used a custom schema.
+
+*Impact* +
+If your application parses {es}'s JSON logs, update it to support the new ECS
+format.
+====
+
+
+.{es} no longer emits deprecation logs or slow logs in plaintext.
+[%collapsible]
+====
+*Details* +
+{es} no longer emits a plaintext version of the following logs:
+
+* Deprecation logs
+* Indexing slow logs
+* Search slow logs
+
+These logs are now only available in JSON.
+
+Server logs are still available in both a JSON and plaintext format.
+
+*Impact* +
+If your application parses {es}'s plaintext logs, update it to use the new ECS
+JSON logs.
+====
+
+// end::notable-breaking-changes[]