[SPDK] [Release] 21.04: ZNS NVMe bdev, PMR, ADQ initiator, RPM

On behalf of the SPDK community I'm pleased to announce the release of SPDK 21.04!

This release contains the following new features:

- ZNS NVMe bdev: Added support for zoned namespaces to the NVMe bdev module. This builds on the work to support zoned namespaces in the NVMe driver in the previous release.
- NVMe PMR: Added support for the Persistent Memory Region feature to NVMe driver.
- NVMe-oF ADQ: Added support for Application Device Queues (ADQ) to the NVMe-oF TCP initiator.
- RPM: Added script for building SPDK RPM packages. See https://spdk.io/doc/rpms.html.

Users updating from the previous release should note that a large number of deprecated APIs have been removed. The process for deprecation has been formalized in https://spdk.io/doc/deprecation.html, where a list of deprecation notices for future releases can be found.

The full changelog for this release is available at:
https://github.com/spdk/spdk/releases/tag/v21.04

This release contains 730 commits from 49 authors with over 29k lines of code changed.

We'd especially like to recognize all of our first time contributors:

	Chengqiang Meng
	Dantali0n
	G Balaji
	Jaylyn Ren
	Klaus Jensen
	Krishna Kanth Reddy
	Maciej Szulik
	Matthew King
	Thanos Makatos

Thanks to everyone for your contributions, participation, and effort!

Thanks,
Tomek