updates for 2.3.0-rc1 release

Update release notes, and bump version numbers in various files.

Signed-off-by: David Brown <[email protected]>

Author: d3zd3z

README.md

@@ -18,7 +18,7 @@
 [travis]: https://travis-ci.org/mcu-tools/mcuboot
 [license]: https://github.com/mcu-tools/mcuboot/blob/main/LICENSE
 
-This is MCUboot version 2.3.0-dev
+This is MCUboot version 2.3.0-rc1
 
 MCUboot is a secure bootloader for 32-bits microcontrollers. It defines a
 common infrastructure for the bootloader and the system flash layout on