CHANGELOG: set Xen 4.15 release date

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>

CHANGELOG.md

@@ -22,7 +22,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
    no longer be built per default. In order to be able to use those, configure needs to
    be called with "--enable-qemu-traditional" as parameter.
 
-## [4.15.0 UNRELEASED](https://xenbits.xen.org/gitweb/?p=xen.git;a=shortlog;h=RELEASE-4.15.0) - TBD
+## [4.15.0](https://xenbits.xen.org/gitweb/?p=xen.git;a=shortlog;h=RELEASE-4.15.0) - 2021-04-08
 
 ### Added / support upgraded
  - ARM IOREQ servers (device emulation etc.) (Tech Preview)