Update version to 17.2.0-rc6

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
diff --git a/VERSION b/VERSION
index 4637b0c..aa131fa 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-17.2.0-rc5
+17.2.0-rc6