Update changelog for 8.12.2 (#2139)

* Backport changelog for 8.12.1

* Add changelog for 8.12.2

docs/changelog.asciidoc

@@ -1,6 +1,28 @@
 [[changelog-client]]
 == Release notes
 
+[discrete]
+=== 8.12.2
+
+[discrete]
+==== Fixes
+
+[discrete]
+===== Upgrade transport to 8.4.1 https://github.com/elastic/elasticsearch-js/pull/2137[#2137]
+
+Upgrades `@elastic/transport` to 8.4.1 to resolve https://github.com/elastic/elastic-transport-js/pull/83[a bug] where arrays in error diagnostics were unintentionally transformed into objects.
+
+[discrete]
+=== 8.12.1
+
+[discrete]
+==== Fixes
+
+[discrete]
+===== Fix hang in bulk helper semaphore https://github.com/elastic/elasticsearch-js/pull/2027[#2027]
+
+The failing state could be reached when a server's response times are slower than flushInterval.
+
 [discrete]
 === 8.12.0