release: Zephyr v3.1.0-rc3

Release tag for Zephyr v3.1.0-rc3.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
diff --git a/VERSION b/VERSION
index e11542d..667e868 100644
--- a/VERSION
+++ b/VERSION
@@ -2,4 +2,4 @@
 VERSION_MINOR = 1
 PATCHLEVEL = 0
 VERSION_TWEAK = 0
-EXTRAVERSION = rc2
+EXTRAVERSION = rc3