2.15 and forward getting-started docs should point people to edge CLI

[campbel]

Move to use -edge as the current script gives this warning.

*******************************************
* This script is deprecated and no longer *
* installs stable releases.               *
*                                         *
* The latest edge release has been        *
* installed. In the future, please use    *
*   run.linkerd.io/install-edge           *
* for this behavior.                      *
*                                         *
* For stable releases, please see         *
*  https://linkerd.io/releases/           *
*******************************************

[wmorgan]

Ooops, thank you!