3.12.1 release notes

(cherry picked from commit 2deb52b)

Author: michaelklishin

release-notes/3.12.1.md

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+RabbitMQ `3.12.1` is a maintenance release in the `3.12.x` [release series](https://www.rabbitmq.com/versions.html).
+
+Please refer to the upgrade section from the [3.12.0 release notes](https://github.com/rabbitmq/rabbitmq-server/releases/tag/v3.12.0)
+if upgrading from a version prior to 3.12.0.
+
+This release requires Erlang 25 and supports Erlang versions up to `26.0.x`.
+[RabbitMQ and Erlang/OTP Compatibility Matrix](https://www.rabbitmq.com/which-erlang.html) has more details on
+Erlang version requirements for RabbitMQ.
+
+
+### Minimum Supported Erlang Version
+
+As of 3.12.0, RabbitMQ requires Erlang 25. Nodes **will fail to start** on older Erlang releases.
+
+Users upgrading from 3.11.x (or older releases) on Erlang 25 to 3.12.x on Erlang 26
+(both RabbitMQ *and* Erlang are upgraded at the same time) **must** consult
+the [v3.12.0 release notes](https://github.com/rabbitmq/rabbitmq-server/releases/tag/v3.12.0) first.
+
+
+## Changes Worth Mentioning
+
+Release notes can be found on GitHub at [rabbitmq-server/release-notes](https://github.com/rabbitmq/rabbitmq-server/tree/v3.12.x/release-notes).
+
+### Core Server
+
+#### Bug Fixes
+
+ * Classic queues v1 (CQv1) that had a backlog of messages stored by 3.9 and earlier versions
+   could run into an exception during queue index recovery after an upgrade to 3.10.x or any later series.
+
+   CQv2 and queues without a backlog were not affected.
+
+   Contributed by @gomoripeti (CloudAMQP).
+
+   GitHub issue: [#8458](https://github.com/rabbitmq/rabbitmq-server/pull/8458)
+
+
+### OAuth 2 Plugin
+
+#### Enhancements
+
+ * The plugin will now accept JWT tokens without a `scope`. Such tokens would only be useful when the plugin
+   is used exclusively for authentication and not authorization.
+
+   GitHub issue: [#8391](https://github.com/rabbitmq/rabbitmq-server/issues/8391)
+
+
+
+## Dependency Upgrades
+
+None in this release.
+
+
+## Source Code Archives
+
+To obtain source code of the entire distribution, please download the archive named `rabbitmq-server-3.12.1.tar.xz`
+instead of the source tarball produced by GitHub.