readme: Update for next dev release

Bump version described in README.md to a development version of the next
release.

This also cleans up some stray release note files that were part of the
previous release, but didn't get removed.

Signed-off-by: David Brown <david.brown@linaro.org>

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
+This is MCUboot version 2.4.0-dev
 
 MCUboot is a secure bootloader for 32-bits microcontrollers. It defines a
 common infrastructure for the bootloader and the system flash layout on

boot/zephyr/VERSION

@@ -1,5 +1,5 @@
 VERSION_MAJOR = 2
-VERSION_MINOR = 3
+VERSION_MINOR = 4
 PATCHLEVEL = 0
 VERSION_TWEAK = 0
-EXTRAVERSION =
+EXTRAVERSION = dev