Move the identity changelog to the directory expected by the pipeline. (#953)

See release pipeline failing: https://dev.azure.com/azure-sdk/internal/_build/results?buildId=616379&view=logs&j=630dead4-e82d-55e1-e443-1204aff8eed9&t=a860a438-9be2-5317-d14b-bab172a8d9b6

This makes it consistent with all the other libraries, like core and storage:
https://github.com/Azure/azure-sdk-for-cpp/blob/master/sdk/core/azure-core/CHANGELOG.md

Author: ahsonkhan