delete outdated upgrading docs (kubernetes#1462)

This branch deletes the "Upgrading Multicluster in Linkerd 2.9" and
"Upgrading to Linkerd 2.10: ports and protocols" documentation from the
2.12 docs' "Tasks" section. This documentation refers to outdated
Linkerd versions, and shouldn't be present in the 2.12 documentation.

These docs will still be available in the 2.9 and 2.10 docs sites, but
should be removed from the 2.12 docs.

Closes kubernetes#1456

linkerd.io/content/2.12/tasks/upgrade.md

@@ -464,7 +464,8 @@ before you can upgrade to Linkerd 2.10.
 Additionally, there are two changes in the 2.10.0 release that may affect you.
 First, the handling of certain ports and protocols has changed. Please read
 through our [ports and protocols in 2.10 upgrade
-guide](../upgrading-2.10-ports-and-protocols/) for the repercussions.
+guide](../../../2.10/tasks/upgrading-2.10-ports-and-protocols/) for the
+repercussions.
 
 Second, we've introduced [extensions](../extensions/) and moved the
 default visualization components into a Linkerd-Viz extension. Read on for what
@@ -575,7 +576,7 @@ multicluster instructions](../installing-multicluster/).
 
 Users who installed the multicluster component in Linkerd 2.8.x and wish to
 upgrade to Linkerd 2.9 should follow the [upgrade multicluster
-instructions](../upgrade-multicluster/).
+instructions](../../../2.9/tasks/upgrade-multicluster/).
 
 ### Ingress behavior changes