Documentation: Fix 4.12 release notes, improve checklist

Since I seem to forget to remove "Upcoming" in the release notes
every single time, and others might as well, fix it here and also
tell future release managers to take care of that.

Change-Id: I8ffe34f25e954621bdd635e115a8b1a914df9c27
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41337
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

Documentation/releases/checklist.md

@@ -67,6 +67,9 @@ be more frequent than was needed, so we scaled it back to twice a year.
       ask for testing.
 - [ ] Test the commit selected for release.
 - [ ] Update release notes with actual commit id, push to repo.
+- [ ] Create new release notes doc template for the next version.
+- [ ] Fill in the release date, remove "Upcoming release" and other filler
+      from the current release notes.
 - [ ] Run release script.
 - [ ] Run vboot_list script.
 - [ ] Test the release from the actual release tarballs.

Documentation/releases/coreboot-4.12-relnotes.md

@@ -1,5 +1,5 @@
-Upcoming release - coreboot 4.12
-================================
+coreboot 4.12
+=============
 
 coreboot 4.12 was released on May 12th, 2020.