Update 2.9.0 -> 2.9.1 as current candidate version (#7442)

Release 2.9.1

Author: atalman

tools/scripts/generate_binary_build_matrix.py

@@ -77,7 +77,7 @@
 
 
 CURRENT_NIGHTLY_VERSION = "2.10.0"
-CURRENT_CANDIDATE_VERSION = "2.9.0"
+CURRENT_CANDIDATE_VERSION = "2.9.1"
 CURRENT_STABLE_VERSION = "2.9.0"
 CURRENT_VERSION = CURRENT_STABLE_VERSION