Add changes for edge-20.2.3 (#4113)

## edge-20.2.3

This release introduces the first optional add-on `tracing`, added through the
new add-on model!

The existing optional `tracing` components Jaeger and OpenCensus can now be
installed as add-on components.

There will be more information to come about the new add-on model, but please
refer to the details of [#3955](#3955) for how to get started.

* CLI
  * Added the `linkerd diagnostics` command to get metrics only from the
    control plane, excluding metrics from the data plane proxies (thanks
    @srv-twry!)
  * Added the `linkerd install --prometheus-image` option for installing a
    custom Prometheus image (thanks @christyjacob4!)
  * Fixed an issue with `linkerd upgrade` where changes to the `Namespace`
    object were ignored (thanks @supra08!)
* Controller
  * Added the `tracing` add-on which installs Jaeger and OpenCensus as add-on
    components (thanks @Pothulapati!!)
* Proxy
  * Increased the inbound router's default capacity from 100 to 10k to
    accommodate environments that have a high cardinality of virtual hosts
    served by a single pod
* Web UI
  * Fixed styling in the CallToAction banner (thanks @aliariff!)

CHANGES.md

@@ -1,3 +1,32 @@
+## edge-20.2.3
+
+This release introduces the first optional add-on `tracing`, added through the
+new add-on model!
+
+The existing optional `tracing` components Jaeger and OpenCensus can now be
+installed as add-on components.
+
+There will be more information to come about the new add-on model, but please
+refer to the details of [#3955](https://github.com/linkerd/linkerd2/pull/3955) for how to get started.
+
+* CLI
+  * Added the `linkerd diagnostics` command to get metrics only from the
+    control plane, excluding metrics from the data plane proxies (thanks
+    @srv-twry!)
+  * Added the `linkerd install --prometheus-image` option for installing a
+    custom Prometheus image (thanks @christyjacob4!)
+  * Fixed an issue with `linkerd upgrade` where changes to the `Namespace`
+    object were ignored (thanks @supra08!)
+* Controller
+  * Added the `tracing` add-on which installs Jaeger and OpenCensus as add-on
+    components (thanks @Pothulapati!!)
+* Proxy
+  * Increased the inbound router's default capacity from 100 to 10k to
+    accommodate environments that have a high cardinality of virtual hosts
+    served by a single pod
+* Web UI
+  * Fixed styling in the CallToAction banner (thanks @aliariff!)
+
 ## edge-20.2.2
 
 This release includes the results from continued profiling & performance