Updating azcore & azidentity changelog (Azure#15423)

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- [ ] The purpose of this PR is explained in this or a referenced issue.
- [ ] The PR does not update generated files.
   - These files are managed by the codegen framework at [Azure/autorest.go][].
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- [ ] Updates to [CHANGELOG.md][] are included.
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[Azure/autorest.go]: https://github.com/Azure/autorest.go
[CHANGELOG.md]: https://github.com/Azure/azure-sdk-for-go/blob/main/CHANGELOG.md

sdk/azcore/CHANGELOG.md

@@ -1,6 +1,17 @@
 # Release History
 
-## v0.19.0
+## v0.20.0 (Unreleased)
+
+### Features Added
+
+### Breaking Changes
+
+### Bug Fixes
+
+### Other Changes
+
+
+## v0.19.0 (2021-08-25)
 
 ### Breaking Changes
 * Split content out of `azcore` into various packages.  The intent is to separate content based on its usage (common, uncommon, SDK authors).
@@ -25,7 +36,7 @@
   * This required some changes to the internal interfaces for core pollers.
 * The core poller types have been improved, including more logging and test coverage.
 
-## v0.18.1
+## v0.18.1 (2021-08-20)
 
 ### Features Added
 * Adds an `ETag` type for comparing etags and handling etags on requests

sdk/azidentity/CHANGELOG.md

@@ -1,6 +1,19 @@
 # Release History
 
-## v0.9.3 (Unreleased)
+## v0.11.0 (Unreleased)
+### Features Added
+
+### Breaking Changes
+
+### Bug Fixes
+
+### Other Changes
+
+## v0.10.0 (2021-08-30)
+### Breaking Changes
+* Update based on `azcore` refactor [#15383](https://github.com/Azure/azure-sdk-for-go/pull/15383)
+
+## v0.9.3 (2021-08-20)
 
 ### Bugs Fixed
 * `ManagedIdentityCredential.GetToken` no longer mutates its `opts.Scopes`