Update installation instructions for gh copilot cli

[kelly-sovacool]

@kopardev
The gh copilot extension has been deprecated in favor of a new implementation, so the installation instructions in this repo are out of date. See details here: cli/cli#12218 (comment)

[kelly-sovacool]

Apparently the user interface is different as well. Unfortunately gh copilot no longer works as it used to, but copilot cli doesn't have all the same features.

[kelly-sovacool]

interestingly, I can only get the new copilot cli to work when disconnected from the VPN.

[kelly-sovacool]

Direct link to the new installation docs: https://docs.github.com/en/copilot/how-tos/set-up/install-copilot-cli