revert upstream change to depend on opencensus >0.12 because it is not released yet

Author: petrem

contrib/opencensus-ext-azure/setup.py

@@ -43,7 +43,7 @@
     install_requires=[
         'azure-core >= 1.12.0, < 2.0.0',
         'azure-identity >= 1.5.0, < 2.0.0',
-        'opencensus >= 0.12.dev0, < 1.0.0',
+        'opencensus >= 0.11.0, < 1.0.0',
         'psutil >= 5.6.3',
         'requests >= 2.19.0',
     ],