Initial 3.9.15 release notes (backport #4413)

release-notes/3.9.15.md

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+RabbitMQ `3.9.15` is a maintenance release in the `3.9.x` release series.
+
+Please refer to the **Upgrading to 3.9** section from [v3.9.0 release notes](https://github.com/rabbitmq/rabbitmq-server/releases/tag/v3.9.0) if upgrading from a version prior to 3.9.0.
+
+This release requires at least Erlang 23.2, and supports Erlang 24. [RabbitMQ and Erlang/OTP Compatibility Matrix](https://www.rabbitmq.com/which-erlang.html) has more details on Erlang version requirements for RabbitMQ.
+
+
+## Changes Worth Mentioning
+
+Release notes are kept under [rabbitmq-server/release-notes](https://github.com/rabbitmq/rabbitmq-server/tree/v3.9.x/release-notes).
+Contributors are encouraged to update them together with their changes. This helps with release automation and a more consistent release schedule.
+
+### Core Server
+
+#### Bug Fixes
+
+ * Stream delivery rate could drop if concurrent stream consumers consumed in a way
+   that made them reach the end of the stream often.
+
+   GitHub issue: [#4371](https://github.com/rabbitmq/rabbitmq-server/pull/4371)
+
+
+### Management Plugin
+
+#### Bug Fixes
+
+ * `rabbitmqadmin` now correctly loads TLS-related keys from its configuration file.
+
+   Contributed by @fwolfsjaeger.
+
+   GitHub issue: [#4408](https://github.com/rabbitmq/rabbitmq-server/pull/4408)
+
+
+## Dependency Upgrades
+
+ * Ra was [upgraded to `2.0.6`](https://github.com/rabbitmq/ra/compare/v2.0.4...v2.0.6)
+
+
+## Source Code Archives
+
+To obtain source code of the entire distribution, please download the archive named `rabbitmq-server-3.9.15.tar.xz`
+instead of the source tarball produced by GitHub.