Doc/releases: Fix coreboot 4.15 release notes

coreboot 4.15 has just been released, so it's neither "upcoming" nor
"planned" anymore.

Change-Id: I287e40deec5877764e511885e3268b606caff597
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59121
Reviewed-by:  Felix Singer <felixsinger@posteo.net>
Reviewed-by: Jason Glenesk <jason.glenesk@amd.corp-partner.google.com>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/releases/coreboot-4.15-relnotes.md

@@ -1,7 +1,7 @@
-Upcoming release - coreboot 4.15
+coreboot 4.15
 ================================
 
-The 4.15 release is planned for November 5th, 2021.
+coreboot 4.15 was released on November 5th, 2021.
 
 Since 4.14 there have been more than 2597 new commits by more than 219 developers.
 Of these, over 73 contributed to coreboot for the first time.