Include upgrade instructions in `run.linkerd.io/public/install`

[scottcarol]

This is a feature request that came from discussion in #3018 in the Linkerd2 repo.

Currently, when a user runs curl https://run.linkerd.io/install-edge | sh, the confirmation text does not mention linkerd upgrade. This can be confusing as the "Now run:" spelled out in the confirmation text will not work for users if they already have Linkerd installed.

run.linkerd.io/public/install should have these changes:

When a user runs curl https://run.linkerd.io/install-edge | sh

• Check for linkerd in the user's path when they download Linkerd
• If linkerd is NOT in the user's path, add link to upgrade docs at the end of output
• If linkerd IS in the user's path, add link to upgrade docs before the "Now run:" section.

Linkerd was successfully installed 🎉

Upgrading? https://linkerd.io/2/tasks/upgrade/

Installing for the first time? Add the linkerd CLI to your path with:

  export PATH=$PATH:$HOME/.linkerd2/bin

[rest of output stays the same]