client.notify deprecation warning on 0.12 dev

[bellini666]

I run neovim nightly, started getting this warning since 0.11 release (as dev became 0.12)

- WARNING client.notify is deprecated. Feature will be removed in Nvim 0.13
  - ADVICE:
    - use client:notify instead.
    - stack traceback:
        /Users/bellini/.local/share/nvim/lazy/copilot.lua/lua/copilot/client.lua:322

 ~
- WARNING client.request is deprecated. Feature will be removed in Nvim 0.13
  - ADVICE:
    - use client:request instead.
    - stack traceback:
        /Users/bellini/.local/share/nvim/lazy/copilot.lua/lua/copilot/client.lua:312

[andrewwillette]

I opened a PR to resolve it!