Update ci triggers

[mayakacz]

What this PR does / why we need it: Updates CI triggers

Which issue this PR fixes (use fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged):

• Run on main instead of master. Updates Rename 'master' branch to 'main' #140
• Run on (and so not block) markdown changes, like README: update description to maintained by Tailscale #138 and CODE_OF_CONDUCT: update to contributor covenant #139

Special notes for your reviewer:

[mayakacz]

I don't know how to fix the meta issue of running CI on this

[davidsbond]

I think the reason the CI isn't running now is because this part of the workflow spec will only run for pull requests that have a branch named main. I think what you want instead is:

on:
  push:
    branches:
    - main
   pull_request: {}

This will trigger the CI for all pushes to main and all pull requests that are opened/synced etc.