3.8.21 release notes

(cherry picked from commit 01f4c06)

Author: michaelklishin

release-notes/3.8.21.md

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+## RabbitMQ 3.8.21
+
+RabbitMQ `3.8.21` is a maintenance release that addresses a regression introduced in `3.8.17`.
+All users are recommended to upgrade to this release.
+
+### Obtaining Packages
+
+RabbitMQ releases are distributed via [GitHub](https://github.com/rabbitmq/rabbitmq-server/releases), [Cloudsmith](https://cloudsmith.io/~rabbitmq/repos/),
+and [PackageCloud](https://packagecloud.io/rabbitmq).
+
+### Erlang/OTP Compatibility Notes
+
+This release [requires Erlang 23.2](https://www.rabbitmq.com/which-erlang.html) and [supports Erlang 24](https://blog.rabbitmq.com/posts/2021/03/erlang-24-support-roadmap/).
+
+[Provisioning Latest Erlang Releases](https://www.rabbitmq.com/which-erlang.html#erlang-repositories) explains
+what package repositories and tools can be used to provision modern Erlang versions.
+
+
+## Upgrade and Compatibility Notes
+
+See the [Upgrading guide](https://www.rabbitmq.com/upgrade.html) for general documentation on upgrades and
+[RabbitMQ change log](https://www.rabbitmq.com/changelog.html) for release notes of other releases.
+
+If upgrading from a`3.7.x` release, see [3.8.0 release notes](https://github.com/rabbitmq/rabbitmq-server/releases/tag/v3.8.0)
+upgrade and compatibility notes first.
+
+If upgrading from a `3.6.x` or older [release series](https://www.rabbitmq.com/versions.html), first upgrade
+to [`3.7.27`](https://github.com/rabbitmq/rabbitmq-server/releases/tag/v3.7.27) and then to this version.
+
+
+## Getting Help
+
+Any questions about this release, upgrades or RabbitMQ in general are welcome on the [RabbitMQ mailing list](https://groups.google.com/forum/#!forum/rabbitmq-users)
+and [RabbitMQ community Slack](https://rabbitmq-slack.herokuapp.com/).
+
+
+## Changes Worth Mentioning
+
+Release notes are kept under [rabbitmq-server/release-notes](https://github.com/rabbitmq/rabbitmq-server/tree/v3.8.x/release-notes).
+Contributors are encouraged to update them together with their changes.  This helps with release automation and more
+consistent release schedule.
+
+### Core Server
+
+#### Bug Fixes
+
+ * Queues that had messages with per-message TTL in them could fail to recover their indices
+   after a restart.
+
+   GitHub issues: [#3284](https://github.com/rabbitmq/rabbitmq-server/pull/3284), [#3272](https://github.com/rabbitmq/rabbitmq-server/discussions/3272)
+
+
+
+## Dependency Upgrades
+
+No dependency changes in this release.
+
+
+## Source Code Archives
+
+To obtain source code of the entire distribution, please download the archive named `rabbitmq-server-3.8.21.tar.xz`
+instead of the source tarball produced by GitHub.