chore: update release notes for new release (#214)

Author: coryan

CHANGELOG.md

@@ -1,6 +1,14 @@
 # Changelog
 
-## v0.22.x - TBD
+## v0.23.x - TBD
+
+## v0.22.x - 2020-03
+
+* fix: workaround a libstdc++ bug on `std::random_device` (#208)
+* cleanup: use same clang-tidy in all repositories (#206)
+* chore: remove EnvironmentVariableRestore (#190)
+  **NOTE:** this class (like all types and functions in the
+   `google::cloud::testing_utils` namespace) was not intended for public use.
 
 ## v0.21.x - 2020-03