Bump aks-preview version to 19.0.0b21 for rollback feature release (Azure#9557)

Author: InduSridhar

src/aks-preview/HISTORY.rst

@@ -11,6 +11,9 @@ To release a new version, please select a new version number (usually plus 1 to
 
 Pending
 +++++++
+
+19.0.0b21
++++++++
 * `az aks nodepool get-rollback-versions`: New command to get available rollback versions for an agent pool.
 * `az aks nodepool rollback`: New command to rollback an agent pool to the most recently used configuration (N-1).
 

src/aks-preview/setup.py

@@ -9,7 +9,7 @@
 
 from setuptools import find_packages, setup
 
-VERSION = "19.0.0b20"
+VERSION = "19.0.0b21"
 
 CLASSIFIERS = [
     "Development Status :: 4 - Beta",