release: Zephyr 3.1.0-rc1

Release tag for Zephyr v3.1.0-rc1

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
diff --git a/VERSION b/VERSION
index a2138c0..825f90f 100644
--- a/VERSION
+++ b/VERSION
@@ -1,5 +1,5 @@
 VERSION_MAJOR = 3
-VERSION_MINOR = 0
-PATCHLEVEL = 99
+VERSION_MINOR = 1
+PATCHLEVEL = 0
 VERSION_TWEAK = 0
-EXTRAVERSION =
+EXTRAVERSION = rc1