Add v6.0.4 changelog

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

Author: kenhys

CHANGELOG.md

@@ -2,6 +2,70 @@
 
 About the past changelog entries, see [CHANGELOG v4](CHANGELOG-v4.md) [CHANGELOG v5](CHANGELOG-v5.md) instead.
 
+## Release v6.0.4 - 2026/MM/DD
+
+### News
+
+* Update bundled Fluentd to v1.19.3
+* Update bundled gems
+* msi: fixed a bug that bundled ruby version was out-of-date.
+
+### Core component
+
+* ruby v3.4.9
+* jemalloc v3.6.0
+* OpenSSL 3.6.1 Windows
+* OpenSSL 3.0.8 macOS
+* gems
+  * fluentd v1.19.3 (update)
+  * msgpack 1.8.0
+  * oj 3.16.11
+  * webrick 1.9.2
+  * openssl 3.3.0
+
+### Bundled plugins and gems
+
+* aws-partitions v1.1150.0
+* aws-sdk-core v3.234.0
+* aws-sdk-kms v1.110.0
+* aws-sdk-s3 v1.208.0
+* aws-sdk-sqs v1.101.0
+* aws-sigv4 v1.12.1
+* elasticsearch v8.19.2
+* fluent-diagtool v1.0.5
+* fluent-plugin-elasticsearch v6.0.0
+* fluent-plugin-flowcounter-simple 0.1.0
+* fluent-plugin-kafka v0.19.6 (update)
+* fluent-plugin-metrics-cmetrics v0.1.2
+* fluent-plugin-fluent-package-update-notifier 0.2.3
+* fluent-plugin-obsolete-plugins v0.2.2
+* fluent-plugin-opensearch v1.1.5
+* fluent-plugin-opentelemetry 0.5.2
+* fluent-plugin-prometheus v2.2.2 (update)
+* fluent-plugin-prometheus_pushgateway v0.2.1
+* fluent-plugin-record-modifier v2.2.1
+* fluent-plugin-rewrite-tag-filter v2.4.0
+* fluent-plugin-s3 v1.8.4
+* fluent-plugin-sd-dns 0.1.0
+* fluent-plugin-systemd v1.1.1
+* fluent-plugin-td v1.2.0
+* fluent-plugin-utmpx v0.5.0
+* fluent-plugin-webhdfs v1.6.0
+* mini_portile2 v2.8.9
+* prometheus-client v4.2.5
+* rdkafka v0.21.0
+* ruby-kafka v1.5.0
+* systemd-journal v2.1.1
+* td-client v3.0.0
+* webhdfs v0.11.0
+
+On Windows
+
+* fluent-plugin-parser-winevt_xml v0.2.8
+* fluent-plugin-windows-exporter v1.0.0
+* winevt_c v0.11.3 (update)
+* nokogiri v1.19.2
+
 ## Release v6.0.3 - 2026/03/27
 
 ### News