release: Zephyr 4.3.1

Set version to 4.3.1.

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>
Signed-off-by: Chris Friedt <chris@fr4.co>

Author: jhedberg

VERSION

@@ -2,4 +2,4 @@ VERSION_MAJOR = 4
 VERSION_MINOR = 3
 PATCHLEVEL = 1
 VERSION_TWEAK = 0
-EXTRAVERSION = rc1
+EXTRAVERSION =