Update the changelog with v6.2.3 information (#17612)

CHANGELOG.md

 # 6.3.0 (unreleased)
 
+# 6.2.3 (2018-06-17)
+
+### Features / Enhancements
+* **AuthProxy**: Optimistic lock pattern for remote cache Set. [#17485](https://github.com/grafana/grafana/pull/17485), [@papagian](https://github.com/papagian)
+* **HTTPServer**: Options for returning new headers X-Content-Type-Options,  X-XSS-Protection and Strict-Transport-Security. [#17522](https://github.com/grafana/grafana/pull/17522), [@kylebrandt](https://github.com/kylebrandt)
+
+### Bug Fixes
+* **Auth Proxy**: Fix non-negative cache TTL. [#17495](https://github.com/grafana/grafana/pull/17495), [@kylebrandt](https://github.com/kylebrandt)
+* **Grafana-CLI**: Fix receiving configuration flags from the command line. [#17606](https://github.com/grafana/grafana/pull/17606), [@gotjosh](https://github.com/gotjosh)
+* **OAuth**: Fix for wrong user token updated on OAuth refresh in DS proxy. [#17541](https://github.com/grafana/grafana/pull/17541), [@redbaron](https://github.com/redbaron)
+* **remote_cache**: Fix redis. [#17483](https://github.com/grafana/grafana/pull/17483), [@kylebrandt](https://github.com/kylebrandt)
+
 # 6.2.2 (2019-06-05)
 
 ### Features / Enhancements