wheel.yml: Do not build Python 3.6 wheels

[tcuongd]

cmdstanpy >=1.0.1 (requirement introduced #2148) requires Python >= 3.7: https://github.com/stan-dev/cmdstanpy/releases/tag/v1.0.1

Wheel build test: https://github.com/facebook/prophet/actions/runs/2275140778

[tcuongd]

@WardBrian I see a lot of packages requiring Python >= 3.7 nowadays, so this change shouldn't be controversial

[tcuongd]

Hmm running into rate limiting issues again while downloading cmdstanpy to build the wheel...

 WARNING:cmdstanpy:Cannot connect to CmdStan github repo.
  Cannot connect to github.
  Cannot connect to CmdStan github repo.
  HTTP Error 403: rate limit exceeded

[WardBrian]

Does this action have a GitHub token set in the environment? I believe that raises the rate limit considerably

[tcuongd]

It doesn't, and I think I need elevated permissions to add one. Good to know though -- I'll raise that.

I just hit re-run on the specific job and everything ran fine (we have cmdstanpy-only wheels now, woohoo!). We might just have to keep hitting retry if the wheel build fails for now.