Pin azure-keyvault-administration dev requirement (Azure#16260)

azure-keyvault-administration fails the mindependency test because its dev_requirements.txt includes the master branch's azure-storage-blob, which requires a later version of azure-core. Azure-keyvault-administration doesn't need the latest blob API, so a simple fix is to pin an azure-storage-blob version having a compatible azure-core requirement.

Author: chlowell

sdk/keyvault/azure-keyvault-administration/dev_requirements.txt

@@ -2,8 +2,8 @@
 -e ../../../tools/azure-sdk-tools
 -e ../../core/azure-core
 -e ../../identity/azure-identity
--e ../../storage/azure-storage-blob
 -e ../azure-keyvault-keys
 -e ../azure-mgmt-keyvault
 ../azure-keyvault-nspkg
 aiohttp>=3.0; python_version >= '3.5'
+azure-storage-blob==12.6.0