Sync 1.21 release notes from master

Change I5e6f7d48, which is not supposed to be backported to 1.21
(see Id442e6f) included unrelated changes to RELEASE-NOTES-1.21,
which are applied to REL1_21 in this patch.

Other release notes were apparently added in error and have been
deleted from master in I6a51e3ee, so they are deleted here.

Change-Id: Iea528109cf8a23d9da2bd370ce7e4e50fba4c6db