[ANNOUNCE] NSPR 4.21 Release

[Kai Engert]

The NSPR 4.21 release is now available. The hg tag is
NSPR_4_21_RTM. The source tar file can be downloaded from
https://ftp.mozilla.org/pub/mozilla.org/nspr/releases/v4.21/src/

NSPR 4.21 contains the following changes:
- Use MAP_SHARED for read-only file mappings on MacOS and Android.
- Changed prbit.h to use builtin function on aarch64.
- Test program build fixes.
- Minor fixes to support unified builds.
- Update library copyright notices.
- Removed Gonk/B2G references.

For details, please refer to the list of related bugs:
https://bugzilla.mozilla.org/buglist.cgi?resolution=FIXED&query_format=advanced&product=NSPR&target_milestone=4.21

Thanks to all contributors to this release.
Kai