Update version for v9.0.0-rc1 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index 07e31fc..4f5843a 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-8.2.90
+8.2.91